Hi,
The way it is done is not in the diameter but in the rotor thickness.
Yes i think the GL 1000 is the same diameter but thicker, it will need grinding down.
The other rotor choice is from the CB750K....it has 2 rotors one is the same size as the 400 and the other (i think the return rotor) is thicker.....will also need to be ground down. and the pump body machined to fit etc.
I make a larger rotor for the 400 pump, it will give around 33-35% off the top of my head increased oil flow. not pressure.
Kevin