Can a derated 5hp VFD run a 5hp motor? In other words, if the OP bought a "5hp" 3-phase VFD and hooked it up to single-phase power, could they assume they would get roughly 2.8hp out of the 5hp motor? If the OP is only looking for a little more power, that 2.8hp might be about perfect.

Plus, I imagine they could run a 5hp motor at 2.8hp for longer periods without worry of overheating it.