Toby,
I think I have these HPs figured out now (at least for my rifle anyway). It takes a reduced powder load (as compared to a similarly weighted sabot round) to spin them accurately.
Here's what I did......my typical load is a 348gr aerotip PowerBelt or a 350gr QT sabot over 95gr of 777. In order to shoot a 300gr XTP into the same spot accurately, I have to drop the load to the 80 to 85gr level. Well, after doing just that this weekend, I threw in a 348gr Lead PowerBelt HP with the 80gr of powder. Worked like a charm! <img src=icon_smile_big.gif border=0 align=middle> Three shots yielded a 2" group at 50yds - all centered around the "X". It would be my guess that a 90gr Pyro Load should do about the same for you.
"Every moving thing that liveth, I give unto you as meat" (Gen 9:3)
Trust God..........but keep your powder dry!