You are considering only the fan speed as the variable to increase the discharge from it.One more parameter that can increase the fan discharge is its inlet air pressure and if it is greater,greater will be the discharge and so maximum transformer capacity should also increase.
At the risk of repeating the same information....
If the motor load increases beyond it's rating for ANY reason whatsoever, the overload protection will operate and shut the system down. That's what the protection is for after all.
So why should I uprate the recovery transformer to cover for abnormal operation outside the rating of the drive system and for which adequate protection exists?
Next you'll be telling me that I should uprate the motor just in case......the cables to it, the switchgear......gets a bit silly doesn't it?
Note also that, at the highest output from the drive, the recovery transformer is at its lowest loading anyway.
You do understand that, don't you?