Although I run 284RUM, as opposed to 338RUM, I'm thinking that maybe I ought to chime in. After hearing numerous people [including a 6.5/284AI BR guy that I was shooting along side, at a public range] talk it up and recommend that I try it, I finally did. I really wasn't expecting the results that I got. In order to achieve the same velocities and pressures that 95gr of RL-25 produced (that's the absolute max charge, using 139gr SSTs), it took an incredible 110gr of the 33 (It was given that # designation because the 338 is what it was specifically desgined for).
The only way to get 110gr into a [284] RUM cartridge is to fill it to the top of the neck, tap it numerous times so that the powder settles down to maybe 1/2 way down the neck, then top it off once again with additional powder. Obviously, seating of the bullet will then yield a highly compressed charge. Now you may (probably DO) think that sounds totally nuts (it is actually MY fist foray into such territory), but pressures were closely monitored, and DID NOT exceed those generated by 95gr of 25.
Now 95gr of RL-25 IS a few grains above the optimal accuracy charge. But my hopes that the equivalent charge (110gr) of RL-33 might be able to produce better group sizes simply did NOT happen.
In summery, RL-33, which also seems to produce much more smoke than 25, did not show ANY advantages for the 15 additional grains of powder "hoggishness" that it necessitates. I'm not all that surprised that it is too slow, even for 250gr 338 cal bullets. Maybe it would work OK in something with a 35-40" barrel?
P.S. Those results were produced in a 26" barrel - I will be stepping up to 32" in about 2 months.