[snellemin]

The 4s will be more efficient on paper, but that is extra weight in batteries over a 3cell. Save some weight and go faster with the use of cheat mode for example. You will be surprised as how fast that motor will go with an increase in timing.
For my sensorless slotless 2 pole motors I use 0-8 degrees of timing. For the goofy roar slotted motors I use 30 degrees to see some fun action.

Personally I'm thinking going with the 550 Ballistic with the 14mm rotor option for my next GTP.