I say the 1515 is fine. When I had one in my truggy it never broke a sweat and had all the power I could ask for. The only reason I went to a 1518 was because of the Castle clearance sale.
Yes a 1515 can do the job, but a 1518 or 1520 can do it with better efficiency for the same power (geared for the same top end), meaning longer runtime.
Inferno VE MMv3 NEU-CC 1515/1Y 4S "Flying machine"