On 18 cells, it is possible to achieve a longer runtime, if you are not too enthusiastic on the throttle. You have got more energy in totall.
The gearing.... well.... For bashing use, i would not gear it higher than a 14 pinion with the 51 spur.. (gearing ratio of 17.8:1 in 2nd) about 45 mph/65 km/h speed) for speedruns you might ad a tooth, but keep an eye one the temps! And i suggest to use a fan on the esc.