My G37 is flex fuel tuned with ECUtek. You just need the sensor of choice and wire it in by bypassing your EVAP sensor. I'm also NA and got about 15 more WHP when at or above 75% ethanol; but I have a fairly long laundry list of bolt-ons.
Link to G37 boards to my dyno
I got my tune done at Topspeed Motorsports in Alpharetta. I was the first VHR that they flex fuel tuned so they are still working out some little things but otherwise it's pretty awesome. The gains when going from 93 OCT to E85 is very noticeable.