86% Omastar with Rock Throw/Ancient Power

keep it the way it is, rock slide one would probably be better. He can be a 2 tier collector item in the future.

Legacy Omastar has two niches where it is strongest, Lapras and Fire Pokemon - STAB + SE against Lapras and resists its ice moves on top of it, and its typing gives it double resistance to fire, making it a CP efficient prestiger in those situations.

If you're attacking into either of those Pokemon you could use a powered up Omastar but there are plenty of options too. It's not useless if it's powered up but I believe it has more value/usefulness with lower CP as a prestiger.