You probably won't notice much of a difference. The stock tune is set to turn down the power to keep it at stock levels. If you did feel a difference, I don't think it would be worth the cost increase. Putting 100 octane shouldn't hurt anything so feel free to give it a try.
True, IF you are only going to tune it to utilize the flexfuel sensor and aren't going to increase power levels. If you are also going to increase power levels you need to upgrade to the 42 lb injectors, the OEM injectors simply cannot flow enough fuel for an E85 tune with higher than OEM airflow (boost). Heck, they barely flow enough for a standard non-E85 tune.
Maybe not even increased power. I have 60lb injectors on my Grand National and if I were to run E-85, with my current setup I would need to go to 72lb injectors. Once I rebuild my short block for my GN, I'm converting to E85. My goal is around 600rwhp and low 10s quarter mile and for that I will need bigger fuel lines, a double pump and 120lbs injectors. Since my GN is fun only, I could care less about gas mileage.
