Regarding 0.8 vs. 1.0mm wire : I don't have any experience of high-power 3-phase welders at settings > 200 Amps. For your 280 Amp machine, with 10 voltage steps, I suppose position 8 is about 220 Amps. Using 1.0mm wire would pass more current, and deposit more metal for a given wirefeed speed. Could you maybe find someone to let you have a few metres of 1.0mm to test if this improves performance at the higher current ? The 0.8mm roller will probably work OK, and if the tip is worn it may also accept thicker wire for a brief test.
Since the machine is old and has been unused for a long time, I would still check out if the wirefeed is performing at it's best.
For the motor: blow out carbon dust, clean the commutator, check brush length and spring pressure.
On the gearbox, remove old grease and replace with new.
The torch - maybe the liner is blocked with rust? Replace the liner, or test with a new Eurotorch.
Since the machine is old and has been unused for a long time, I would still check out if the wirefeed is performing at it's best.
For the motor: blow out carbon dust, clean the commutator, check brush length and spring pressure.
On the gearbox, remove old grease and replace with new.
The torch - maybe the liner is blocked with rust? Replace the liner, or test with a new Eurotorch.