First of, the RUM can handle 25% (or 25 grains) more powder than the rem mag, thereby adding around 10% more velocity to any given bullet, resulting in 25% more "hitting power".
I suppose that most of those who do go ultra do so because they are looking for big bore size, and/or big bullet weight. But apparently I am one of those individuals who was blessed (or more likely, cursed) with the "need for speed" mentality (note my user name). I maxed out (actually, max + 1.5) on the mag at 75gr of RL-19, without my bolt going tight. Velocity at 15ft was 3680fps, + or - 5fps on 80% of the rounds fired. That probably equates to 3700 at the muzzle. Despite not having a tight bolt, I WAS very nervous firing those rounds.
The one over-charge that I did have (not sure how it happened), gave me a reading of 3875 at 15', with a very tight bolt! The amount of RL-19 that my figures indicate would have been needed to achieve that velocity, is 83gr! It must have been a really compressed load. THAT put put a scare into me, so it was at that point that I decided to chamber-up to something that can handle that much (and considerably more) of a charge, without the bolt going tight. From THAT perspective, doesn't 4000 seem reasonable for the RUM, especially since the required charge would probably still be a good 10-15gr below the max capacity (100gr) of the RUM.
And barrel burnout in only 100 rounds? Why do you think THAT low? The 220 runs similar/greater velocities. Albeit, it IS known to be a barrel burner, but with modern steel formulations, I'm thinking it should do 500-1000 rounds before accuracy goes out the window. I'm looking at the 7RUM that I'm building as the 220's "big brother".
EDIT: Just like the 220 can basically "vaporize" prairie dogs, I want to vaporize ground hogs!