Need some help with exhaust sizing question
 

Im not a fan of the borla thats on my car sounds OK but.. current: Z1 headers to 2.5 test pipes to 2.25 borla cat back on my supercharged car pushing 634whp at sea level.

I don't understand if they went down in pipe diameter but I have someone interested in buying the used borla. I really liked the greddy quality and sound though I have no idea the sound on an SC car. Its on national backorder.

A lot of exhausts go from the y pipe to a 3" collector then into say a 2.5 b pipe. would that setup be worse for flow than the 2.25 true dual borla that's already on the car?

My personal options I was looking at are:
ISR ST with their Y
Greddy GT cant find it in stock
FI 2.5 non resonated but I really dont want to fork out 1500+ and wait
Maybe the OBX race but may be stupid loud
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