Which set up is the most efficient...
 

All motors on 5s. for a 1/8 buggy geared 44/16

1. 1512 2.5 2050kv 37925 rpm
2. 1512 1.5y 1850kv 34225 rpm
3. 1512 3d 1700kv 31450rpm

I am not looking for blinding speed...I run on a short track w/ my nitro buddies and am looking to prove to them that my set up is not only efficient but fast.

Since they are all geared the same with the same voltage, they should all be about the same efficiency. You aren't changing the "load". Personally, I'd get the 1.5y or 2.5d and gear for the speed you want.