Hi Ropata, I hope I have not steered you in the wrong direction?
My experience with the RPC + transformer performs well. I can't comment on how efficient it is - nothing gets very hot, the power bill hasn't changed much, the machines work well. Of course, the more devices in the system, the lower the efficiency & reliability are going to be. Mark is right about the buzzing noise, though it is still quieter than my RPC - I do have them located in a seperated part of the garage (along with the air compressor) - with the dividing door closed all is good.
Big? Perhaps that depends on how much room a person has. My 6kva transformer has a bit smaller foot print than my 5HP RPC. The spec sheet should identify what the dimensions are and you could decide if that would be workable in your situation.
For multiple machines and easy controls, it is hard to beat the RPC option.
Reading about your efforts to find a transformer, I consider myself very fortunate to have found a, tested working, 240-575V, 3 ph, 6kva in close driving distance for $100. I certainly thought it was a good deal at the time and acted on it immediately (I didn't haggle on price and got there before he changed his mind).
Good luck, David