It is pointless using an auto transformer, as the resistance losses would kill it off too easily.
Remember, you are using a 48v to 240v/120v conversion, so for whatever the thickness of the secondary winding may be, you should have the corresponding mass of wire on the primary in the ratio of the voltage.... ie if you have 10mmsq of wire in the secondary, you should have at least 80mmsq of wire in the primary ( 240v) It is ok to go as low as 35mmsq of primary if your duty cycle is low... but can you imagine it as a auto transformer? The whole thing would need to be wound at 50mmsq, and it would still get hot under decent load, as there would be 100 turns of wire in there.
So the low losses in coupling would be more than wiped out with all that copper.. if you could fit it in..... not a useful step forward I don't think.
You could perhaps change the winding thickness when you got to the secondary part..... but the isolation would still be nice to have..... if I were to do it, a total rewind would be on the cards, as you have a lot of core there.
The losses will still be about twice for the same power as a toroid one ( twice as much steel)
............oztules