Just based off some light tech reading, I also suspect the piston change alone may be what fixes it, but I want to see how the softer springs feel on a bumpy track.

I was running the stock 25 pulley, and trying to keep a 2mm split all day, so trued down the fronts twice until that final where the 42's I had were 2mm larger than the fronts had become, in the final with 40/42 wear was quite even front to rear.

Not real sure of what the Mugen pulleys relate to in 1:1 drive ratio on the 5, with the setup sheet saying to run a 2mm split with 25 pulley, I'd guess that is 1:1?, meaning 26 tooth pulley gives 1:1 with same diameter tyres front and rear?.