Hello flex-fuel curious members! Just thought I'd chime in here since I've had a DT Flex-Fuel kit on my '05 LGT for about 6 months now. Yes, this kit hijacks the intake air temp sensor, but it does seem to work just fine. It's a very noticeable difference in power between E85 and 93. Like m sprank quotes above, +40whp is what I got by using E85 from a close to stock setup. The tune changes to whatever combination of gasoline and ethanol is in the tank. It's been pretty much set-it and forget-it for me.
The only issue I have noticed is that cold starting can be bad. I talked to my tuner and he said that there isn't really any way to tune a solution to this with my old '05 ECU. He suggested adding a couple gallons of 93 when it gets cold, especially if you've still got summer blend e85 in the tank.
My car has never failed to start or anything, just has taken a lot of cranking to get started, then runs rough for maybe 30 seconds. For instance, I had 70% ethanol in the tank yesterday morning (38 degrees) and the cold start was very difficult. I added a few gallons of 93, and today it started much easier (55% ethanol, 37 degrees). That's really my only annoyance so far though.