...can I do away with gas checks for wheelweight alloy up to say, 2000fps if powdercoated?
The concensus seems to be that tumble coated bullets where the entire bullet including the base, is coated, the gas check no longer serves any purpose. However, the hardness of the bullet still is a factor in how fast a cast lead bullet can be pushed.
For instance, in my misspent youth, I experimented with how soft a bullet could be when fired at .44 Magnum velocities. I found that when I used a near-pure lead alloy, my recovered 429421 Keith-type bullets had mysteriously developed rifling marks on the forward (nose), of the bullets, indicating that they were "squashing" into more barrel-shaped objects than before they were fired. In other words, they were losing their shape because the lead was too soft for that velocity.
When I was (some years ago), obsessed with accuracy with cast lead bullets in bottle-neck, modern cartridges (30-06, etc.), The Cast Lead Bullet Association's (CBA), match data (and my results) indicated that best target accuracy was obtained at just a tad over 1,000 fps. with relatively hard bullet alloys (Linotype or heat treated).
Therefore, if seeking 2,000 fps., your alloy will I assume, have to be quite hard to maintain accuracy...there have been some posts at the Cast Boolit website that indicates it is possible with powder coat, but I do not remember what alloy they were using, or if they in fact listed the alloy in their posts.