Re: THA thing you did to your ammo to help your precision is ?
I know you asked for reloading suggestions only but I' ve got to ask you a couple things on your rifle before I chime in.
Has your action been bedded, how do you torque your screws down onto your stock, barrel floated, and are you satisfied you have a good crown on barrel.
You said your groups really open up when you approach med or upper end velocity is why I asked the rifle questions. That would not sit well with me. If I had a 25-06 that I had to run at 1/2 speed, well you may as well shoot a .243
I will tell you that the best advice I ever got 40 years ago was keep everything consistant ALL THE TIME.
I buy components in larger qty's with same lot numbers, that includes brass, primers, powder and bullets.
Do you know what twist your barrel is - has a lot to do with what weight bullets to use.
Another thing to consider is your shooting form , and trigger pull. Are you comfortable with the parallax on your scope. Last thing I would suggest is, I quit shooting and doing load evaluations at 100 yds many years ago. I finally got it through my thick skull I am not a bench rest shooter. Move out to 200 or 300.
I have a rifle shooting Bergers that will shoot at 200 as good as it will at 100.
From the amount of fliers you are getting I would say it is either miss matched components or something other than your ammo.
Good luck and don't give up.