I recently upgraded my stock Thunder Tiger Sparrowhawk XT from Brushed to Brushless.
The Brushless system I have used is the Castle Creations Sidewinder SCT 3800kv Motor ESC combo.
I ordered it from tower hobbies.
I have tried a 3s 40C Zippy Lipo and what I can say is it is pretty damn fast and way more powerful than before.
Here is a Video of it doing some speed Runs