The muffler of the tme is slightly different.
If you change the full exhaust you will really have a good improvement in power on the evo,
especially after 5000rpm!
Some people on evo vi had a full exhausty system fitted and they get from 280bhp to 305bhp whithout other modification.That's also because when you fit a complete exhaust on the evo you have less back pressure and usally the turbo pressure raises by itself a little.
Depending on the exhausts,you can loose some low end torque,but if you only fit a cat back system you will have better torque at lower rpm and better power.
If you fit the downpipe with decat you will have another 5 or 10bhp.
Whith the full exhaust system even if you have a little less low end,the car is much faster!
I would suggest the magnex if you want a good complete exhaust at a good price,around 550£.
The hks hyper muffler with the extension kit,as it include the turbo elbow,the part before the downpipe,that it's only included in the hks and the power engineering exhaust and would probably give even more power.But it's a little bit more expensive,around 950£.
The new apex hayabusa,it's good quality,nice look and really light!
If you just want a cat back,you can fit the blitz spec r as it's straigh and will be a little bit louder than other catback.
I have a full supersprint system on my evo and it's not that loud,so if you want to have some sound on your evo go for a small silencer exhaust!