David,
My apologies for not getting in print what has been ruminating in my head most of the day. Had some chores and weather is getting ready to move in so I wanted everything in place ifin' the creek rises.
To start you got a real hot shot there and from what I understand was a real head turner for a bit around the time I was hatched. You didn't specify if it was the Revolver or the Contender that you were getting your results with but I can try to offer a couple of solutions. One off the top of my head is going to standard LPP's, Alliant's web site has 2400 loads in .44 using standard CCI 300's and I would start there. I know you are using a necked down case and smaller diameter/ lighter projectile so the use of a magnum primer is overkill IMHO. Another aspect not mentioned is barrel use or how much have you used it. Pressure issues can show up in newer barrels because of tool marks from manufacture. Through use these marks are worn away creating free bore and pressures will drop. The opposite can happen with hot loads and the bore becomes crazed where tiny cracks form in the bore and "grip" the bullet excessively.
Second, root around for a copy of Modern Reloading 2nd ed. by Richard Lee. It has .357-44 B&D loads listed, all using LPP and none use 2400. Two powders that I use regularly are listed and like very much, H110 and H4227. You may not achieve the screaming velocities you mentioned but over 1600 fps in a handgun is respectable in any ones opinion.
As for brass, if you haven't already looked into Starline I highly recommend them, I don't know if you will get more use from pristine unused brass but you do get to determine it's lifespan unlike range pickup that you have no idea who has done what to it. I can't belive I almost forgot this but check your Case lengths, after reworking that brass and necking it down some trim work is/maybe needed. Long cases will cause pressure issues too.
Hope some of this rambling helps!