Greg Gregorich said:
he can have cpu bottleneck or inefficient pcu
It doesn't really matter. If he would bottleneck with a 1060 6GB he was probably already bottlenecking with a 1060 3GB, and he'd still end up with better performance when he upgrades his CPU (or better resale value when he sells the 1060 down the road). And if by PCU you mean PSU, if it could handle a 1060 3GB it can handle a 1060 6GB.
I am having trouble sometimes with GPU temp spiking to mid to high 70s
temperatures in the 70s aren't really anything to worry about.