My rears aren't filled, but I do have steel wheel weights on the back...80# per side.
You probably won't have any problems as problems go, but I find it much more comforting to have the box blade mounted to the 3PH when handling snow. Sure it is lighter in its 'fluffy' state, but when you handle volumes of snow with the FEL, it will compress in the bucket quite well and you'll end up handling very dense blocks of slush before you know it. As such, I have not doubts you could get to handling 300# and 400# loads of snow in the bucket near the end of the job (pile making process).
Not that it is the end of the world, but I hate it when the rear end is bouncing everywhere. The rear ballast makes a big difference.