I think the existing boxes can be modified to make them fully straight through when the valves are open. Think this is what Matt/Conaero had on his GS? But obviously you can't put them back to standard after that.
The level of noise you can get from the car will also depend on whether you have the US or Euro spec manifolds. US spec means primary cats on the manifolds and then secondary cats in the centre section. So even if you remove the secondary cats, you still have another set of cats. A second hand pair of Euro manifolds goes for 700-800 quid and then it's about a grand in labour to fit them in a pig of a job. You can then have just one set of sports cats. I want to do this but can't justify the outlay...