zpb, are you really asking that question? LOL

The although I don't have a 1515, the 2.5d should at least equal to power of the 8XL, give or take a couple hunderd kv. I've heard nothing but good things about the 2.5d. Just going by kv values and my experiences with brushless in general, I'd go with the 2.5d just to have the flexibility of 4S or 5S lipo. I think the 1Y is too hot for extended running on 5S. If 4S (or its equivalent) is what you are sticking with and performance is on your mind, yo could opt for the 1Y, which I would think is on par with 7XL performance.

Is the 8XL what you are running now?