Your not going to gain anything especially if your mid pipe is only 2.5 If your car is at the power level where you need a 3in, most people dump it becasue the biggest problem willl be trying to get a 3in pipe to fit over the axles/control arms and clear the gas tank. Go with the 2.5 you will thank me later.
I got 3 inch piping up and over Wheel wels and 3 1/2 inch tips, no biggie. Take the time, bend it right and it will fit, just a lot of patience! hahahahaha
but like he said-you really dont want to go 3 inch from headers back, unless you have the power to support it! 2 1/2 inch H-pipe is more than enough! Thats what i got!