I read the Hodgdon website about them a while back. I'm not sure if it's a larger charge pellet or a different, concentrated formulation. Their website does warn against the use of more than two pellets though. If it increases velocity, there will also be increased felt recoil. More energy going out the barrel equals more energy moving backwards as well.
777 loose is expensive enough; I can only imagine what these new "magnum" pellets are gonna set folks back. I use 110 gr. of loose 777 with a 250 gr. bullet, get the same velocity and it's less expensive to boot. Can't say I'll be spending the cash for the "magnum" pellets.