here's my 2 cents:
My tc3 is good, but I am looking to up grade. The fastest guys at my track are running xrays, and yesterday, I got second to an xray who had almost a lap advantage. But im not about to go buy a new car, when i still have people asking for my setup. I was thinking about switching to a tc4, but listening to this thread, it sounds like tunability just isnt there.