Actually what the after market companies tell you is entirely correct, what your told is "a bunch of crap" unfortunately.
The less back pressure you have on your exhaust the more HP and the faster your car will be. Needing back pressure is ONLY on a 2 stroke engine. This was though to be the case about 20 years ago but when we actually woke up and knew something about exhaust flow and how a 4 stroke engine works we have since realized that is not the case.
Here are some random googles:
http://www.uucmotorwerks.com/html_product/...etorquemyth.htm
http://auto.howstuffworks.com/question172.htm
Even on the most high strung engines if you just put on a header and basically no exhaust at all it will be disgustingly loud but it will make more power. On a honda lets say, with a 1.8L GSR (B18C) engine, the optimum exhaust system for racing is a header and full 3" exhaust with no cat no muffler no resonator. It will make more top end power than any other setup, (other than just an open header but generally not allowed in most racing circuits).
Unfortunately this is a non-debatable area so please don't come back to me with this mid range torque answer etc etc.. thats a bunch of BS. Nobody cares about mid range torque unless your towing a trailer. So put a trailer on your Porsche then come complain to me and I'll listen. Otherwise the smallest amount of back pressure you can put on your car is the best for top end HP which is where you want it to go fast.
Exhaust systems are designed with every other factor considered except HP. Meetings todays ever stricter noise and emissions regulation is what stipulates an exhaust systems size bends muffler design etc. Not HP. That is the last consideration unless the companies are directly violating it (Lambo, Ferrari, CGT etc..) and don't care.
If you remove any restriction (the cats are one of the largest sources, second are the horrible manifold designs used by Porsche) you will make more power. Goto a dyno with your secondary cat bypass and you will make some small measure of hp increase. It may not be a lot and may make your car sound horrible in the process so the increase may not be worth it. If you were full out racing you would have a header straight pipe to basically the smallest muffler (or no muffler) that your racing series would allow because that would make the highest amount of peak hp which is exactly what you want.
Not trying to sound rude or upset anybody etc, please don't take it that way. I have worked on a race team and races small displacement n/a cars for about 8 years now and we made 269whp from a 2.0L normally aspirated and have tested every exhaust combination you can think of. I have built street cars for guys who street race hardcore and want fast cars to guys who race in auto-x etc. I dyno cars and change parts at the dyno to measure increases in power. Hell my dad races a Nissan Maxima and made 292whp normally aspirated, his highest HP run was with headers straight pipe no cat and no muffler on a VQ35DE which is a 350Z motor in a 4th gen maxima.
Watch this youtube clip of a 986 race car http://www.youtube.com/watch?v=r1-hZ2VLKiQ...ted&search=
Read the comments by the clip poster about what exhaust system he is running on a race car, now ask yourself why he would be doing that?
Regards,