The 6.5 should be a good pick. I started with a 5.5 in my T4 which was a bit much and went down to the 7.5 which may be a little on the slow side. While I haven't had the gtb combo long, I've had the older ss5800 for a long time and the only maintaining I've done is oiling the bearings. The only replacement part I've used is a new rotor and endbell but that was by choice to get the larger endbell bearing and upgraded rotor. From what I've heard, if you end up overheating the Velociti motor you may end up weakening the magnet and needed to replace. If your club doesn't follow ROAR rules fully, the sintered rotor is a nice upgrade. I've only just started using it and I have noticed the motor seems to have a bit more power and definately more drag brake and stronger breaking. Even seem to run cooler.
I totally recommend the jump to brushless and if you combine with Lipo(again if your club allows), life will get easy. I just finished a 10 minute run using a Orion Platinum and only put 2400 Mah back in.