Howdy,
I'd just like to throw my hat into the ring here.
The two headers aren't US and Euro, they're Euro 3 compliant and Euro 4 compliant. Euro 3 affected cars produced between 2001 and 2006, with Euro 4 coming into affect from 2006.
https://en.wikipedia.org/wiki/European_emission_standards#Emission_standards_for_passenger_cars. Manufacturers are typically trying to meet the next standard several years before it becomes a compliance requirement, so will often experiment on engines already in the market to see if they will be viable for use in the next generation of cars or if they will need a new platform. This is the real reason why the usage of the headers seems somewhat random. Typically hot engines perform better due to
https://en.wikipedia.org/wiki/Thermal_efficiency but hotter engines result in more
https://en.wikipedia.org/wiki/Nitrogen_oxide which is bad. The way around this is to implement catalytic converters, but catalyst efficiency is temperature dependent so placement matters
https://en.wikipedia.org/wiki/Catalytic_converter#Placement_of_catalytic_converters .
Our chassis was never designed to accommodate the F136 engine and the exhaust design demonstrates that. An important thing to remember with exhausts is that in terms of power they're extremely forgiving on bad design because they're under pressure and will always perform their function of letting gasses escape. This is important to keep in mind when thinking of exhaust options for this car because every option is literally a compromise due to packaging.
The Euro 3 header kept the cats hot by using a double skinned design; this is not a performance design even if it does have a performance implication. The reality is that air is a fantastic insulator so in cold start conditions the double skin keeps the hottest gasses getting to the cats to get them to operational temperature as soon as possible.
https://www.researchgate.net/public...tem_design_based_on_heat_transfer_computation . The engineers used the dry sump ground clearance to package the cats under the engine to allow the headers to be longer.
The Euro 4 header needed to make a more stringent cold start requirement so the solution was to package the header closer to the exhaust ports. This trades off performance by shortening the headers with emissions performance. This design has an affect on the Larini headers as the heat shielding design is different.
The black line on this image is where the collectors on the Larini headers end - they're in effect a shorty style header. Larini would've done this so that they can sell the products as a proper bolt in product and compromised ideal performance for fitment. The changes for heat shielding on the Euro 4 headers are what dictates the overall shape, and therefore design, of the Larini headers.
The point I'm trying to make is that there is no 'best' set of headers, they all serve a particular purpose. That said, as this is a forum for enthusiasts we all consider best to be related to a combination of sound and performance, so with that in mind - the Euro 3 headers are the best you can do with minimum fabrication.
In reality, the Euro 3 headers are still too short - our engine is low revving (yes, it is low revving, fight me IRL). This isn't one I can give you a formula or a justification for, this is just an 'in my experience comment'.
The below units are made by a shop called Best Mufflers in Sydney
https://www.bestmufflers.com/bshop/Custom_Exhaust_Systems/Maserati_Custom_Exhaust_System.htm I called him when I started looking into an exhaust system for my car. He built them on a jig and can make another set. He wanted $7k AUD, which is 3600GBP today. Unfortunately that is way more than I can justify for my car but if I was choosing a new off the shelf set of headers I'd pick those over the Larini set. The downside is you will need to get someone to fabricate the mid-section.
I'm getting a set of the Euro 3 headers modified to be longer as the cost is more reasonable and I get to customise the sound a bit further. The car was supposed to be at the exhaust shop today but that has been delayed for a few more days... The current plan is to cut off the collectors on the Euro 3 and extend the headers into an 8-1 collectors under the sump. We're 50:50 that it will fit, but we won't really know till the headers are on the car and the cutting starts. The CAD seems to show it will fit, but building it in practice is different.
As for the performance part of the question; as I said before exhausts are very forgiving to bad design so changing exhaust components generally only yields 2-5% improvements on naturally aspirated cars. On this car, I believe if you had the money to do some experimenting you could probably see 7-10%, but most of that would just be in going from 4 high cell cats to proper high flow cats. People tend to be disappointed when they change exhaust parts and
only get 5%, but the reality is that is a large improvement for exhaust components. Keep in mind that exhausts don't create power, or make power, what you're doing in practice is allowing gasses to escape more efficiently. That power was already there, so if you flip the way you think about it around it makes a lot more sense: Does my exhaust take up 10% of the engine's power to escape? When you think about it like that it almost seems ridiculous that an exhaust could consume that much energy. Also, keep in mind, 7-10% improvement is a peak improvement, you may see zero improvement in peak power, and depending on how you setup the exhaust you may even lose a bit of peak power. The below graph illustrates my point - peak improvement was 22% on this particular car, but peak power only improved by 16%.
If sound is your main goal, well then no holds barred. Think about what kind of sound you want and go nuts. The engine is a cross-plane crank so you can magnify that characteristic or you can try and minimise it and smooth the sound. Regardless, removing the cats, going to headers of any length, it will sound better. The best set of headers compared to the worst set will at most affect your power output by 1-2%.
The biggest issue affecting after market car parts is grade-A marketing ****. Almost every dyno plot you see will be generic fake nonsense made in marketing departments. Wheel dynos are
not SAE rated and so their numbers are almost always fake. This car sounds nice but it is not making 350 wheel horses
Maybe 290 on a good day.
This dyno plot is straight off the FD website for their ECM chip, this is absolutely fudge factor 9000.
Fabspeed, same deal.
This dyno was measured in Australia from a shop I know. Power is in kW which is ~290 wheel horses.
This my dyno plot over that dyno plot, keeping in mind these were recorded in different cities, years apart, with 50 000km difference between the engines and through a broken diff...
This is a really long winded way of saying don't buy into 'marketing' and 'speaking highly' of car parts. Try to understand a bit about
why something was designed a particular way and then you can make an informed decision. Knowing your end goal will also make things a lot less confusing. But mostly, just have realistic expectations of what you can achieve.
Hope that helps.