Dan, do I understand your current setup is a 1 ph motor? If so, you'll need a 3 ph along with a VFD drive to go variable speed.
Will it make it faster? Well, that depends (don't you love those type answers?). What is diameter of your drive wheel? With a 4" drive wheel and the motor spinning at 3800 rpm you'll have around a 4,000 SFPM belt speed. The 3800 rpm is the same if you're using a 1800 rpm motor with a double RPM jumper, or a 3600 rpm motor and only overspeeding to 3800 RPM. Both about the same, and in use you'll never know the difference if the motor is 1800 rpm or 3600 rpm, except the 3600 rpm motor is less weight and less expensive. Yes, at 1800 rpm there is more torque and power, but at 1800 rpm you're not using anywhere neat all the motor HP anyway, so it's a mute point.
Nope, going to a VFD isn't going to help any with belt slippage. You might try wrapping a layer of rubber tape around the drive wheel - that does help.
Changing to VFD/direct drive is worth the money just for the extra smoothness and speed control. I preached for a couple of year my 3 speed gave me all the speeds I needed..... until I finally built another grinder with VFD. I'm sold! I've got 4 VFD drives running in the shop now, one on a lathe, one on disk grinder, a horizontal grinder, and standard old 2X72 grinder. They're all the cheap Chinese drives - well, the lathe is a Telco, but it's still "made in China", just had to pay extra for the Telco name. It was my first.
Ken H>