since I have the 2.5d on 6s that is what I would say to go with, I wouldnt run the 2.5d with anything less than 5s, you would be disappointed with it on 4s, I get 30 min run times with my Revo using a 5000 6s pack mostly WOT, I never use my brakes...( I view it as a sign of defeat).......lol...I have thought of trying 8s with my set up but I feel I have enough power anymore and it would be wasted...my front end will come up a bit and the front tires ballooning after about 200ft of WOT during a speed run in short grass...and sometimes I get the bug of trying 10s with the 1515/2y but I dont think it will make that much difference...I guess just being able to say I have a 10s Revo.....