IIRC my measurements on the inner pipe after it necks down is around 2"

2.5 would be a difference, the question is.. are you willing to sacrifice the low end for the high end? Being SC'd and running a small pulley, I went to 3" and noticed low end loss. BUT...I had more than enough low end to light them after the fact.

Merely know that there will be a noted difference.

These motors are choked at higher RPM'*. They need all the flow they can get. A 2.5 would be a marginal loss in low-end, prolly nothing terribly noticable. Even on GP'* which have an even more restrictive DP, there are more gains than losses when going to a larger DP. A friend of mine switched to a 2.5" on his GP GT and had great results. Addtionally, the gain in flow will help reduce any KR (yea, the N/A motor can still get KR).

the biggest thing I am looking for is the midrange. I feel that the low end is good enough for me. If I can get a smooth pull that is Ideally what I would like. It is not super important to smoke the tires to me. If I can get a setup where the mid is better and if the I open it up I should see the better mileage here in the mountains. Better flow better mileage right

Yes, you will gain mileage.

power and mileage are the goals. Sound is the next thing but that can always be tweaked