[Herald1360]

300mA off load primary current won't be 75W, it'll be 75VA, of which only some will be watts. Mostly the watts will be the I^2(R) losses in the primary winding resistance.

The TX may only be 85% efficient at full load, but not all those losses will be primary side- there's secondary side losses and magnetic circuit losses as well. At no load, the TX will be 0% efficient- nothing out but some power in.

With the transformer first, you still can't have 500VA out- the no load VA of the variac will come off the total output from the TX.

If there's a worry about overvolting the TX when fed from the variac, most variacs I've met have an option to retap for 0-100% output rather than 0-110%.