Here's some guidelines..

http://www.flowmastermufflers.com/faq.html

http://www.pypesexhaust.com/faq.html


2 inch up to 100 per pipe (UP to 200 hp dual)
2.25 inch up to 160 per pipe (UP to 320 hp dual)
2.50 inch up to 200 per pipe (UP to 400 hp dual)
3 inch up to 300 per pipe (UP to 600 hp dual)

So depending on your hp, -that's something to keep in mind

With your combo, 2.5 is 'around' enough.

-also, X-over is really recommended, for quieter AND better flowing exhaust.

http://www.pypesexhaust.com/xpipes.html


On the 'note' of loud exhaust, I must agree with TOHCan. -My dual 3 inch exhaust is more quiet than the dual 2,5 inch that original was on the car.
-Mainly because my current setup have a x-over, -AND the 'spin tech' mufflers are really better flowing/and more quiet than the flowmasters on the 2,5 inch setup. also a point is the fact that even if my current setup is stainless steel, which has a higher note than 'normal' steel, -it's still more quiet. PLUS this engine has around 450-460 HP vs. the old one that had around 340-350.