I have a 1512 2.5D running on 4S and it will shred the tires off the car if given the chance - I don't think you need a 1515 for a 8lb buggy - not worth the additional weight and more current draw.
Why not look at going for a 1512 3D or 3.5D (1700 and 1500 kv respectively). It would be a cooler running setup and you could get lighter batteries to keep the overall weight down as well.
“Everyone has a right to be stupid; some people just abuse the privilege.”
Woud a neu-1515 2,5D on 6s lipo be a good setup for a 1/8 buggy ?
Depends what you want to get out of it. If you want a good race setup it will have a lot more power than you need in a buggy, so I wouldn't recommend it. But if you want a slightly overpowered basher it will be able to do that.