Only three ways can it hurt:
a) you put too much LR in there, and it leans against the glass and you can't effectively clean it, so you can't effectively look in your tank.
b)something dies in the tank and you have to search through even more rock than your 300 #s
c) the stand you have it on can't hold the weight and the tank comes crashing down spilling saltwater, LR, LS, livestock, shards of glass, and what remains of the tank stand all over the floor; thus ruining carpet/hardwood floor, or creating a real mess for a tiled room.
IMHO the more LR the better. I mean you can never have enough filtration right?