DEFYET,
Not to hijack the thread with a, "I've got one as well", but......
Wayne Jacobson at American Hunting Rifles built me a 450 Rigby based off a CZ 550 Magnum action. I currently shoot the 500 grain Barnes TSX bullets out of mine around 2300 fps (458 Lott velocities). I've never felt the need to drive the velocities up to 450 Dakota or 460 Weatherby levels, although the 450 Rigby can do it.
However, Matt is right about the heavier bullets. That's where the 450 Rigby, and it's voluminous case capacity, shines. A 550 grain bullet with a SD of 0.375, traveling at 2100-2200 fps would penetrate like a you-know-what. The 450 Rigby will handle those bigger bullets with ease. If you have a choice, you might want to serious consider the Woodleigh 550's. My $0.02 of course.......
One day, I'd like to develop a load with North Fork 550 grain Semi Spitzer Bonded Softs and Cup Point Solids out of my rifle. I love the North Forks, as far as accuracy, but I don't love the price. I've never taken an animal with one, but talking to those that have, the terminal performance is phenomenal. I'm not sure however, they'd do any better than a Swift A-frame or Barnes TSX. The big advantage would be the upgrade to the 550 grain weight.
Have fun with that 450 Rigby......