it's not just V3 with legs, it's a much much larger V3 with legs. If it was the same size as the one gai busted, i'd have no qualms with saying AT busts it, but it's not it's the size of a bijuu.The thing is, his susanoo is still V3, its just grew legs. This is V4:You must be registered for see links.
I guess my point is that how will a V3 with legs contribute to having more durability than a V3 without legs? Its durability should be the same in my opinion unless some how adding legs to it will somehow increase the overall durability of the upper half too.
Susanoo can greatly vary in size we've seen madara use a V4 that's incomparably larger than the one sasuke used in that scan, madara's also used a V2 that's barely bigger than a human. Unless you want to argue size isn't a factor, you have to address that a bigger susanoo has more HP and thus can take more damage than smaller ones.