Low end power is my main concern and I am afraid of loosing it with a 3inch exhaust on a car I don't plan on boosting any time soon. What exhaust setup is going to yield the highest exhaust velocity? Not to small not too big is the goal here..

Considering the stock exhaust is 2.5 crush bent, there should be some small gains on a 2.5 mandrel, but would crush bending still be the same as a slightly smaller pipe? For instance, would a 3.0 crush be like a 2.75in mandrel? I wish there were N/A dynos on a 2.75 catback


Also, I plan on putting on a WS ypipe (Or if i can find a set of the latest OBX batch with better fitment and 2.5 y section), and a resonator in place of the cat

 

Stock is actually 2.25"

The smaller the diameter, the higher the velocity.
W/o FI, I'd say stick with 2.5".