Well, I ran 17.5 lipo and 21.5 lipo last night in my Darkside car. This is a small 40'x60' track, but the layout has been the same for the last three weeks.
I hold the fastest lap and most laps in 5min with the 17.5 and lipo, BUT the fastest lap between all options are now less tha 0.15/sec. I have turned the same number of laps with each, and all were the same lap as my fastest TQ run. I did over gear the 21.5 for one heat to see if it would get a little faster so, as usuall there is a sweet spot in each motor for a track with acceleration and speed.
In my opinion, the 17.5/lipo is maybe a tick faster(better acceleration), the 13.5/4 cell middle of the row and the 21.5 probably just a tick off.
But at the club level, I'd run all three, just like VTA and let the racers run what they like. Bottom line with all these, a good chassis and driver trumps the motor battery if similar.
Disclaimer - I've been racing RC cars for 20+ years, I'm not a-main material at a national level, and spend my own cash to race toy cars. I buy what works for me and work very hard to promote rc racing ... helping start a new club here in Colorado when our carpet track closed.
So take it for what is worth (or not) and lets get more cars on the track!
Now if I could get my hands on the Jaco Spec Tire's