Just went thru a test drive with the TMC tune on my 650i GC with a friend. Some quick 2cents comments:
1) Below 2000rpm, the gains are there but not very noticeable. Possible due to the fact that the 650iGC has more than enough low-end torque. My feeling is that if the low-end torque goes any higher than now with boost, the rear axle may have problem putting those torque down to the road......
2) Above 2000rpm, the gains are VERY noticeable. In comfort mode, there is a much firmer push from the back feeling when accelerating and response is fabulous as well. In sport mode, I can only describe it as a major kick in my butt!
I did a pull all the way to 5500rpm and is nothing short of amazing!
3) Gas consumption? Well, I only had it since last night so a bit premature to draw conclusion yet. Let me revisit this after a few weeks.......
4) My feeling is that adding the TMC (or other tunes) to the 650i GC is great if one wanted more power to the car off the straight line but I don't think the 650i GC is lacking in the power department to begin with. I am now more concerned about the 650i GC control of such power. When stepping on throttle even at 75%, my friend and I start to feel that the car starts to sway left and right, like the car is struggling to balance the power of the 2 rear wheels due to minute traction differences. I think a real LSD could really have helped.