The limiting factors on the maximum speed assuming the rotor is balanced and the bearings are capable of higher speeds. Is the internal fan in the motor are how much torque you need at these higher speeds and the surface speed limits of the belt pulleys/ gears.
the motor is a constant torque device up to its rated base speed (4pole /1800rpm) and a constant HP machine past its rated base speed. What this saying is that from minimum RPM to 1800 RPM a 1.5 Hp motor produces 54 inch pounds torque but at 2600 RPM it produces 27 inLbs Torque and at 5400 RPM 18 InLbs. The second issue is that the HP required to turn the internal fan is 8 times higher at 3600 RPM and 27 times higher at 5400 RPM. Somewhere between 3600 and 5400 the motor may stall out simply because the HP required the spin the fan has exceed the motors capability. This is based on the affinity laws which states that the work (HP) required to turn a fan increases by the cube of the speed change.
As for the Pulleys/gears they have Feet Per Minute (FPM) rating that if exceeded they will fly apart.
The main difference between the various drive architectures (Volts/Hz, Sensor less Vector, Flux Vector) is how well they can produce torque at very low speeds, and how well they hold speed accuracy. Also with Flux Vector you will the ability to control Torque.
So set the drive up as if your max speed is 1800 RPM/60Hz. If the drive has an auto-tuning feature auto tune the drive. This is usually found in the SVC and FV drives. Then set the max Hz to say 120 Hz and use the machine. if it appears you have enough torque then up the speed until the machine just starts to slow down and you will be at the drive/motor maximum capability.
The Delta drive is a very good drive. several of what you would think of as the major drive manufacturers are rebranding the Delta drive. The Marathon MAX Plus motor is a top notch motor and is only exceeded by the Black Max and the Blue Max motors . you have some good stuff there.
For what it's worth I have designed, applied and sold drives for over 45 years and I do not have an inverter on my lathe I prefer using the belt pulleys because they are torque multipliers and what you need to hog out metal is torque, high speed is for polishing.