Do you need to water cool erhm a water cooled spindle?!!

[Jonathan]

Just try it and see.

JI think it was Jonathan that said the Chinese spindle were about 80% efficient, so for an 800W spindle surely I'd be looking at a meager 160W heat dissipation

I feel I should clarify this. The spindle is an induction motor, so the efficiency varies somewhat. The efficiency will depend on the load and the speed, so at some point it probably will be 80%. The efficiency will be much lower at low load, but the input power is much lower so overall the power dissipation will not be that great.
I think you're mainly cutting PCBs, so the power required from the spindle is tiny - most of the power is just supplying the losses (bearings, magnetising current etc) so if you just leave the spindle on not cutting anything and see how long it takes to heat up, you'll have a pretty good idea of how long you can use it cutting PCBs.

The situation's very different if you do draw near full power - it will heat up very quickly, but it takes a bit of doing to make a machine for which the spindle power is the limiting factor...