Voltage, Amps, and Heat

[DrKnow65]

Another motor question for you guys,

I have a XXXT I'm running a MM esc, and a cs325700 (5700KV) motor, I'm geared as small as I can get and the motor temps are just acceptable (150') after a good hard run. (2S lipo now, 3S later this month)

I want to step up to a NEU motor (thinking 1509/1.5Y) 2400kv, use 4S lipo's and have a cool running setup.

The question I have is,
I also have a HYPER7 I'm wanting to run brushless, I would like to use the MM/NEU in both setups, what is the smallest NEU I should use in it on 4S? I'd like to have one motor/esc that would work in both (xxxt-hyper) untill the MMM comes out for the hyper7...

The MM esc, is modded with heatsinks on both sides of the fet board (wind tunnel dual fan setup) and runs cooler than ambient in the XXXT no matter what gearing or how hard I drive it, so I'm not worried about the esc.

One NEU, not WAY too big for the xxxt, not WAY too small for the hyper7, 4S in both cars... will a 1509 series be ok? need a 1512 series? will the xxxt steer with a 1515 hanging off the back?