Hey Echd,
I think since your VFD is already set to the rated current of your present 1.5 kW spindle, and that is probably set at or near the highest value the 1.5 kW VFD will give, your power consumption should not change.
Given your comments, I think you are already aware of the 100 / 200 volts class differences/disadvantages.
Also maybe you know that some chinese spindles sold as 2.2 kW spindles are in fact 1.5 kW spindles (and some sold as 1.5 kW are 1.0 kW in real respectively).
But what matters to you should be the current you set in the VFD which should always correspond to the rated current on the nameplate of the spindle (or less if the VFD is not able to provide the rated current).