omoser,
last night a couple of guys were driving 5 cells when training at our club for this weekends LRP Masters. Their comment on gearing was 2-3 teeth bigger pinion for 5 cell use than the same motor+car combination used with 6 cells. One of them was using 8.3 gearing with Orion 3.5T BL motor.
Based on this you might want to try going taller.