need to go slower to go faster??

For 20 minutes you would need to jump up to a 5s 5000 pack to do it consistently. I have run 20 minutes with a 5s 4000 pack in a race, but it is on the very edge. The 1515 2.5d is more suited for a truggy. If you gear down, you are probably going to start running hot as there won't be enough load on the motor.

I would highly recommend a 1512 3d (1700kv) or the 1y (1850) kv for 5s. I pretty much just race with 5s 4000 packs since our mains here are just 15 minutes most of the time.

You only need to gear for 37-40mph.

The only way you are going to get 20 minutes consistently under race conditions (plus warm up laps) is to get the 1512 3d or the 1y and use a 5s 5000 pack. You MIGHT get it with a 5s 4350 pack, but it will be real close.