Well, I went out and got the biggest bullets available for my .454... the CPB WLNGC at 395 grains. I put them over 18 grains of H110, and went out for a little test drive.
The recoil wasn't too bad, it doesn't snap so much with such a big bullet, and the ports on my RB definitely help. At 25 yards, I was shooting about 10" high. Probably my fault from the recoil. But, I was shooting pretty much right on at 50 yards with them. Groups were acceptable, nothing great though, still not used to these thumpers yet.
I also took out some 330 grain LFNGC's out over 11 grains of unique... and that is one great load. Very pleasant to shoot. Gives me about 1050fps out of 6.5" of barrel, and good accuracy. That is my new hunting load. Well, unless I'm going after big critters, then I think the 360 grainers over 22grns of H110 still wins. But those 395 grainers might find their place, because even though it was high at 25 yards, I was still getting about 4 inch groups. Not horrible for their first time out.