When they say reduced performance on single phase, it shouldn't be by much. The main difference is that the input current is shared between 4 rectifier diodes rather than 6. The dc voltage ripple inside is slightly higher but of no concern. It should work identically, with a slight (10-20%) reduction in S1 continuous power rating. I'd be interested to hear what derating they recommend.

I'd be surprised if you got anywhere near 7.5kW continuous shaft power on a machine like this. I'm guessing you bought this for low speed torque as much as anything, at which operating point the input power will be a fair bit less than 7.5kW. Personally I wouldn't bother with the expense of installing 3 phase when you could spend that money on more machine tools....