I've done dozens of installations on various cars over the years and did my 3200 not long after I bought it. Changing speakers will certainly improve sound quality, but there are limitations. My advice would be to approach it in this order:
1. Concentrate on buying the best replacement front speakers you can - most of the sound you want to hear comes from these. As standard they are component speakers (separate driver/woofer in the bottom of the door and separate tweeter mounted higher up). Buy components, not 2-way where the tweeter is mounted in the middle of the speaker. You don't have to replace the factory tweeter if you don't intend to install a bigger sound system further down the line, but I recommend you do so as you should be taking the door panel off anyway. Replacing these is very simple and will make the biggest difference. You will need to either research which speakers to buy, or take the driver/woofer out and take it to your audio retailer to get the correct size as there is very little room to fit and so it must be a shallow speaker. When you fit the replacement driver make sure you insulate the connections with tape as some owners have had issues of the terminals shorting against the metal on the door (the fit is that tight!)
2. If you really want to spend money to no real avail then buy some replacement speakers for the rear quarters. These speakers by design are only for 'rear fill' (to replicate sound bouncing back behind you) and in the standard size will never amount to doing much. I wouldn't bother replacing these and would adjust the fader on the head unit towards the front a little if you find them at all instrusive with the new front speakers running.
3. Rear parcel shelf speakers - even more pointless than the ones in the rear quarters. Again, if you have money to burn and like cramping yourself into small spaces connecting fiddly things then go ahead and replace them.
4. Standard head units, no matter how good/expensive do not contain very good amplifiers. The power outputs claimed are usually very generous and are for peak power, not continuous. They will usually suffice for a simple speaker change, but think of them as entry level. After spending money on good replacement front speakers I would next direct the contents of my wallet towards a separate amplifier. Only at this stage would I consider changing the rear quarter speakers (and disconnecting those in the shelf) as a suitable amplifier will make a considerable difference to sound quality and output/loudness. The amplifier can be mounted in various different places inside the cabin depending on how much of the interior you want to remove. When I'm installing a complete sound system I always remove all seats, consoles and carpets.
5. For those of you still reading, the next stage is where it can get a little more expensive. A subwoofer (mounted in the boot) will provide the low-down bass missing in most car systems and is starting to feature as standard equipment in many cars. The Maserati is a heavy, well built car and so needs something fairly substantial to create the desired effect. This speaker will need it's own dedicated amplifier which can be expensive. The subwoofer and amplifier can be mounted in the boot space, where the spare wheel well can be utilised if you have the standard Maserati can of tyre weld.
Like any audio system you can buy different levels of quality and spend many thousands if it all gets addictive (my Range Rover had over £13k of kit in it!) - buy the best you can afford to get the best effect. If anyone would like some individual advice or even help with buying/installing please let me know.