I had a 7.5 hp okuma howa (since sold and upgraded) that I powered with a 10hp Phase Craft panel and a Toshiba 10hp motor. I was spindle limited to 4500 rpm, and I did not have rigid tap available, but it worked great. Never a problem with power with the spindle load meter in the red ...
If you have rigid tapping and a high speed spindle, I would go with a 15hp converter panel and whatever 15 hp motor you could find. Saves a bunch over a prefab converter.