Well I gave it 100 repeats of each test, I only used a travel 0f 55mm on Z as it trips at 65.
At 3000mm/min in motor tuning, it passed 100%, I started out at Z1 with a feeler gauge on a test plate and after each series it was back at Z1 perfectly.
I tried 4000 in tuning but it was not happy and stalled after two repeats.
So using the 75% of stall setting rule I put it back at 3000.
I did try a couple of simple test cuts and it ran fine, no skips in the G28.1 anyway, also left the THC rate where it was before so that is effectively doubled now I guess and still seems happy, or at least I could not see any sign of see-sawing on the Z axis in the cut.
Did not try G02/3 as not enough knowledge there yet
Now need some jobs to run.
Does not explain why a different screw made any difference though, the only other change was that the nut is now brass and not delrin, don't know yet if thats good or bad though.