I think those that are telling you to abandon Tightgroup with those hot loads and heavy bullets are right.
That powder is just to fast for that bullet weight. H110, LiL'Gun and WIN 296 are much better options.
The Lyman or Hornady manuals don't even list Tightgroup with jacketed bullets and the lead load that Hornady lists max out at 6.6g of tightgroup using a 240g lead bullet.
I know that published max data is normally safe but, your well over that at 9+ grains using 300g bullets.
NRA Life Member, SAF Member