I am only 5 hours in and my medical highlighted obesity as a concern to CASA, so they requested I have my CFI check that I can operate controls and that the W&B check was ok for the aircraft, which is a Warrior as well. Controls were fine as its more my height that gives me issues, and we did the CoG check as requested by CASA.
I am about 140kg and the instructor is 110, so a combined front weight of 250kg. We sat with the CFI and tried a few scenarios based around a fully fueled aircraft. In the end we needed one of the lighter ones in the fleet to give enough room on MTOW, and add 10kg of ballast in the baggage area to bring the CoG back into limits (just). So I can continue flying providing that until I lose some weight I can only fly one particular aircraft that we have a correct calculation on.
At no point was the weight in the seat mentioned by CASA or my CFI.
I would sit down and discuss it with your CFI, as mine was quite helpful in trying to find a solution for me (i.e alternate aircraft, lighter instructor etc).