Luffy's speed advantage will be completely gone in this fight, so he will definitely have more problems than he did with Doffy in that respect. We don't know whether or not Kuma's body was as hard as the Prototype Pacifista, but I'm hesitant to say he is weaker, especially considering he was powerful pirate before the surgery. He might not be super phased by Gear 4, at least, not as much as Doffy was.
I'm also curious as to whether or not Gear 4 can break through Kuma's reflection ability. I assume the Haki would allow it to so though. That'll most likely be Luffy's big advantage in this fight. A point blank Ursa Shock could probably due some damage to Luffy though.
I'll give it to Luffy right now. Kuma's lack of Haki feats puts him at an disadvantage.