Just to follow up with this thread. I have the GTC300 on my car and I dropped around 4 seconds at Willow Springs in my 335i. You will need, as people have said, to counteract all of the rear downforce with a front splitter. You want the splitter/lip combo to run around 4"-6" from the bottom of your bumper. Anymore is just creating drag (unless you have 600whp+).
As for which wing performs best, I will be switching from my 67" GTC300 to 71" GT250. The GT250 will have less downforce, but an extremely less amount of drag. The benefits of the less drag outweigh the benefits of more downforce (which can be increased by AOA). APR only offered the GTC300 at the time I got mine, as I was one of the first cars on either forums running a wing, Tightie would always make fun of it at his track events until he tried it hahahaha.
Either way you will be happy with the product, but the GT250 is the more intelligent of the designs.
-Mike