NO, that would be real bad! Your motor is probably rated 1750 rpms at 60 cycles. So the gear reduction is reducing that down to 1000 rpms with proportionally greater torque at the spindle. Setting the VFD for 300 Hz would make your motor run at 8,750 rpm, and it would likely self-destruct at that speed in a very short time. Also the torque would fall off badly as well. Now if your motor is a 1750 rpm motor the maximum safe speed would likely be 3450 rpm like a standard 2 pole motor would give you. The bearings and all would be fine. So you could set your drive to give 120 Hz for 3450 rpm and then adjust your gear reduction to produce 5,000 rpm, and that would be okay, assuming your spindle is okay with that speed. The torque of course will be lowered proportionally as well. So no free lunch with a VFD. People who really like them simply do work where full motor horsepower is seldom needed. If you need the most power at the spindle gearing is always better.