You have described two different scenarios here, use of a VFD and a change in pulley ratios. Apples and oranges.
A VFD can increase the frequency above the motor base speed, but in doing so the motor loses torque. Torque is directly related to the ratio of voltage and frequency. So in your case the motor is rated for 400V 50Hz, a ratio of 8:1. If you increase the frequency above 50Hz, but have no additional voltage, the ratio, and thereby the torque, drops. The power remains the same however.
Most machines work on torque not power however. A screw compressor relies upon torque to keep pushing the air though the moving annular spaces formed by meshing the screw lobes together. If you lose that torque, you either stall the motor or you must slow down the compressor to keep from stalling, which means less air flow, so the exercise was pointless.
In your case you have elected to use a different pulley ratio to change speed. When using pulley ratios, the motor speed remains constant, the work shaft speed changes but the torque changes at the pulley ratio change. So in speeding it up, the motor is still operating the same, but you have still lost torque at the compressor shaft. So although the compressor is turning faster, it now will require more power(kW) from the motor to replace that lost torque. What you might be seeing is that the compressor design engineer may have selected a slightly larger motor than necessary so that when you changed the ratio, you are still within the FLC rating of the motor. To answer that question on the current draw however, technically your motor should be able to run at THAT current forever without issue.
So now reality comes creeping in. Why would the compressor design engineer not utilize the additional capacity if it simply meant changing the pulley ratio as you did? From a marketing standpoint, it would mean they could sell that compressor as having additional capacity over what it was advertised for. So might there be some reason you are unaware of that prevailed over a marketing advantage? Might it be that the extra "head room" in the motor sizing or belt ratio selection was to allow for the vagaries of supply voltages thought the 50Hz world that it was intended to be sold in? For example you are using it at 400V. But a lot of countries that use 50 Hz are using 380V, which may dip to 360V and still be acceptable. So if your motor nameplate FLC is 10.7A based on 400V, that means that the LOAD current must not exceed 9.6A, because if the voltage drops to 360V, that 9.6A will increase to 10.7A. So in all likelihood what you are doing here is choosing to CONSUME that fudge factor rather than hold it in reserve. That will likely be fine, as long as all other things are perfect.
Now, can you put in a larger motor? Sure! But are you a mechanical engineer specializing in screw compressor design who can predict any other unforeseen mechanical consequences?
