[Patrick]

I run a neu 1515 2.5d (1700kv) on 5s in my e-maxx for racing and a bit of bashing (mainly driving around at full speed on grass). It has a fair bit of power, and you can play with gearing without really affecting anything. I recently got a 6s 4900mah 25C flightpower pack and I've tested my car with that pack twice so far doing mainly high speed runs on grass again. Very fast and powerful and temps of everything was pretty low (motor's only ever warm).
So if you want a powerful motor for 6s my vote is for the 1515 2.5d. The 8xl and 1515 1y (2200kv) will have more rpm so you'll probably be able to get them going faster, but they'll draw a lot of power and temps could be an issue if doing any more than a couple of high speed passes (especially with the 8xl).

For the batteries, what sort of mah packs do you want?
If you only want around 5000mah I say get flightpower packs (4900's are 143mm long and 44mm wide, and the 5000's are 167mm by 48mm, but they are a little bit lower. So depends what fits your car best)
If you want 8000mah then probably max amp packs, but my 10c 8000mah truerc packs do pretty well in 5s. The newer 15c packs should be a bit better, but I haven't heard much about them yet.