The Mill Turn spindle is designed to run at the spec speed of 4750 RPM. However, for those who want higher speeds, a fairly simple and inexpensive upgrade is available which will increase your speeds to the 7000 RPM range while retaining all your original lower speeds.

The current bearings are
TD 5207-2RS 1/P5 LOWER
TD 5206-2RS 1P/5 UPPER

These are double sealed bearings with the seals rated to a maximum of 240 degrees.

For the 7000 RPM range, we suggest the following
TD 5207-RS 1P/5 LOWER
TD 5207-RS 1P5 UPPER
These bearings are single sealed with the same 240 degree heat rating, but with only a single seal which reduces friction and heat by 50%. The bearings would be installed with the seals on the outside of the cartridge.

In order to achieve the higher spindle speeds, it will be necessary to change the drive pulley ratios. A simple solution is to replace the motor pulley with a 6" diameter unit, but it has 2 drawbacks- 1. unless you can find a good quality aluminum one, a cast iron one will be very heavy. 2. The close distance between the pulleys and the difference in diameter would give only about 30% contact area on the driven pulley resulting in potential slippage under load.
The better solution is to use tooth belts which have no slippage issues.
The pulleys would be 18 L 050 on the spindle and 35 L 050 on the motor. This would give you 7000 rpm at 120 Hz.
Note: the 050 means the belt is 1/2" wide. These are sufficiently rated for a 2 HP motor, but you can also go to an 075 belt which is 3/4" wide if you prefer.
The bearing numbers are universal and can be sourced from a lot of suppliers at around 30-40.00 each.
The tooth belt pulleys and proper belts can be sourced here XL037 3/8

The bearing swap can be done in an hour, but the pulleys will need to be bored to fit the spindle and the motor, which you can do on your lathe. You will also need to make an aluminum disc to fit above the spindle pulley to accept the sensor magnet.

Probably 4 hours total time once you have all the components and a cost of around 150.00 in parts.