#15 was never max out...and usually, those setting are for 21.5, 17.5 and 13.5 motor. We adjust the time delay (#14) depends on the straight lenght. We adjust the RPM start (#13) base on the motor TURN. And we adjust the #15 depends on the feel....

I have NEVER max out #15....however, #9 and #12, it is like default (max out) for me (at least for 13.5 17.5 and 21.5 stock motor).

And yes...gearing play the most important parts of these setup. Like I said...with my setup, it has the best power (my opinion and my feels) when I use TALL gear...with a 21.5, it can be 6 or so. However, YOU have to try it, then check the temp of the motor and check lap time compare to others...if you have rooms to GEAR UP....by all means, DO IT...you might be able to gear to 5 or even higher...

I have yet to run 21.5 with BOOST so I can only tell you what I think is the best when running 17.5...and I am just assuming I will use the 17.5 setup on ESC and GEAR UP....my 17.5 motor was running some low to mid 7 FDR..and I will take 21.5 motor can gear up by 1 lower...like low to mid 6 FDR...just my theory.