This is from Remington's website, and talks about the compared energy levels between the .300 RUM and the 300 Wby & Win Mags:


<BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote> The cartridge that launched the Ultra Mag line. And the new standard for big game from deer to elk to caribou. Our field tests back that up. The .300 Remington Ultra Mag delivers a muzzle velocity of 3300 feet-per-second and 2145 foot-pounds of retained energy at 500 yards (while certainly not a typical shot, that’s still more than enough energy for effective performance on elk at that range). Even more impressive is how it fares against the competition: At 200 yards, it delivers up to 18% greater retained energy than the 300 Wby. Mag and up to 27% greater retained energy than the 300 Win. Mag. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>

IMHO significant performance increases. Comparing a .300 Win Mag to a .300 RUM is like comparing a .30-06 to a .300 Win Mag. Yes one can do most of what the other can do, but with big differences in trajectory and retained energy levels.