I think another cell will wake it up a bit !
Since your effective kv is only 525rpm/volt, it is only lightly loaded on 4 cells and that prop.
(By way of comparison, a 16/35/1 with the common 5.2:1 gearbox ratio is 673 rpm/volt and the lunitics here run those on 5 cells and 16x17's. One particular crazy one runs a +5 twisted spinner as well!)
PS: Long shot but just check your esc is seeing 100% tx end point too. Million to one I know but stranger things have happened.