Hi

a decent 3KW spindle will cut through anything as long as the feed rate is correct Gooing to 4.5 KW or higher doesn't make much difference these are usually to drive Long bits or large blades
spindles have efficiency curves, a lot of the 2.2 - 3 kw ones are tuned to perform between 18000 and 22000 rpm, below that speed, they don't have enough torque,

Diamond has a finite working speed if too high it burnishes the Bits exactly the same as Using Carbide bits on granite and polishes the diamond flat.
a lot of the stone bits are low rpm, and some of the spindle i have looked at are High kw but have the torque at lower rpms, its one of the reasons i was considering a belt driven spindle, or a servo as a spindle, i still haven't made my mind up yet.