Call my friend JonB, he sells tons of these systems and can explain the details and kits much better than I. Both of my Corsas are 3".
I wouldn't pay money for a 2.5" system. 3" mandrel bent exhaust don't have much restriction, but 2.5" with 90 degree bends in it is what we used on small block engines back in the 60s and 70s. Anything over 300hp I'd prefer 3" and mandrel bends.
I used Diesel exhaust piping to build my 3" 2.2L turbo exhaust systems. I figure - if you're going to fix the exhaust, might as well do it the best way possible.
Ted