Mark, I've just skim read this thread in one turn,
owch! that must have taken a bit of doing.... well done.

750watt
that should be loads! something not right there... i used a 300w pc supply for quite a while (managed to trip the supply out a couple of times while using a hole saw... besides that its managed ok?)

im now on a cheap 12v 350w china special iv not tripped that one so far

im assuming your saying its to slow for 3mm tooling in wood ?
90% of my cutting is 3mm single flute in acetal between 1mm and 3mm DOC aprox 2000rpm between 200 and 300 feed

at 270kv you would need 48v to get aprox 13000rpm ... might be a tad fast if your using double row A/C bearings unless you have them swishing around in a nice light oil ?
you might have problems with the esc and windings getting hot

id be tempted to make a second spindle with a higher kv rating .... 890kv would give you 10680rpm at 12v