Have you tried getting looser sabots from MMP or Harvestor?
If you've tried so many things I'd send the barrel to T/C and ask for a replacement. I have had good luck with their barrels, MOA accuracy in both of my Omegas up to around 110-115gr 777.
777 is impossible with 209s, you can either fork over the money for a 25ACP conversion or make one yourself by drilling out your breech plug on a lathe or drill press w/a vise. I have both and both work well.
I do not see why YOU should pay to cut the QLA off the barrel if that really is the problem, when T/C should make it right (and likely will). This is NOT a systemic problem with the QLA, I know lots of guys with T/C QLA-equipped barrels and while some maintain accuracy to higher charges than others all are capable of 1-1.5 MOA accuracy.