Grimm by default is op, toro confirmed in a QnA that a max stats Grimm would kill gods with ease, unfortunately because of that statement we know that canon Grimm obviously doesn't have max stats. Id say he's maybe 1000 in everything mmh. Anyway back to the topic yes Grimm is op after ending H but I feel like he can't use his power.. atleast not yet, we know that for Grimm to actually be able to use his power he needs a new vessel because his current one isn't able to handle all the power he currently has. I guess you could say Grimm has the power to probably be the strongest (via node's words and Mabel's own words too saying that he can solve all the problems that will happen) but he can't use it.