Dave,
As you know I have used a 20 ga paradox for some years now. I experimented quite a bit with powders, wads, cases and bullet alloy. The constant throughout was the NEI mold (the WR style mold) and the sizing diameter (a hair under the smoothbore dia). I think my bullet is right around 500gr, from memory.
The issue I encountered (and you will too based on what I remember of that gun), is that as you go higher in velo you have to go harder in bullets to avoid heavy leading/stripping and consequent total loss of accuracy. I made the mistake of putting too much weight on what I read, written by Ross and others, about bullet alloy. The 15:1 or 20:1 alloy everyone says is de rigeur is too soft unless you have very heavy deep ratchet rifling of the H&H type. Your gun (and mine) have less aggressive rifling, they won't spin a bullet that soft unless the velo is very low.
I started out with brass cases by Magtech..they look so romantic but they are a waste of time. First, they are too thin so you need special wads etc. and the bullet kind of floats behind the crimp. Second, they split after two loads.
I ended up using one of the SR powders, a fairly mild load, a plastic obturator over the powder (so absolutely no blow-by), a couple of vege wads, and Rem ribbed cases cut back to 2.5" ...these grip the slug very nicely so no problems with crimping. I also put lube in the bullet groove and shoot it without the hollow point. It doesn't expand because it's quite hard (94:3:3) so no point in HP. The flat nose and the large bore diameter are devastating anyway.
Just work up watching your chrono til the bullets start flying all over the place and then back off. Bronze brush your choke after each pair to keep things consistent. Just remember to do that in the field as well. If you get over 1000 fps, that's enough to kill an eland with one shot (I tried and it does) so no problem with a deer.
I was lucky in that my gun did not require reregulation, it shoots 4" at 100 with the bbls overprinting. I have no idea how your bbls are regulated since I never shot that gun. I would try a bunch of different powder and load combos (you are stuck for bullet choice with NEI cast or the Lyman and Lee sabots I think)
