No kidding, we run 13.5t blinky, and that starts to feel slow. Corner entry set up and maintaining speed through them is so important. There is no room for error, to keep constancy good.

As to gearing, well, that's part of the game, isn't it! But generally;

10.5t=55mm
13.5t=80mm
17.5t=95mm
21.5t=105mm?

These are all ball park roll-outs, track size and style, and motor/timing will determine where you end up. Start conservative and work your way up, temp motor, and go by feel/lap times. Don't be afraid to try a lower roll-out as well. All roll-outs are for running non-boosted 1cell, if boosted, gear to the next motor wind to start, and work up on the timing.