I have shot thousands of 2-3/4 AA hull hand loads through my 20ga 2-1/2 chambered Flues with Krupp barrels. It was loaded with 20/28 at just south of 9k psi.
If you look in the Alliant 20/28 load manual, it is the load listed at 8880psi and 1200fps.
Never an issue.

There is also a 7200psi load at 1200fps with 3/4oz and 20/28 in the manual. It uses a different wad but I've shot some of those with good results as well.

I have a 6400psi load with 11/16 oz of shot in AA hulls that I had Mr. Armbrust test for me years ago if you would like it. Its only running 1075 but breaks clay just fine. I use to shoot it in a damascus Flues 20.