Hi Mike,
I have a 90 C2 and I have been playing with the exhaust to come up with the "right" combination that sounds nice and adds power - gauged by my totally accurate "BUTTDYNO".
Without making recommendations about what you should do - here is what I have found. I have drilled out the airbox, but I don't have a chip.
First, I replaced the primary muffler (the one across the back of the car) with a "cup pipe" - nothing more than a straight pipe. The result was a nice throaty sound and increased throttle response.
Next I replaced the cat with a bypass pipe and it made the car sound really nice, but noticably louder.
Your not going to sneak home late with this pipe on your car ! You will set off alarms going through parking lots with this on. And your neighbors will take an instant disliking to you - especially if you leave for work at 6 AM !
I pulled that off this last weekend.
I tried the cat bypass and cup pipe combo without the secondary and it sounded like crap - REALLY bad. You essentially have straight pipes at this point.
I tried it with the cat back on, cup pipe and no secondary - still sounded like crap.
I am now back to the cup pipe and the cat and I think it maybe the right combination - for me anyway.
I have a secondary bypass pipe being shipped to me right now. I am going to try the OE primary (with the cat in place)and I will post the results.
I hope this helps. You can save a lot of money if you pass on the cat bypass. I think the best combination is in the primary or the seconday being bypassed with the cat in place. Or you can go for a Fabspeed or GHL muffler - $275 vs $800. It's your money
One way or another it looks like I will have a bypass pipe to sell when I get done - I just don't know which one yet.
Tom Sharpes
90 C2 Targa
http://members.rennlist.com/5150targa