For me, more likely "on good/great form" but "in good/great shape", although thinking about it, I'd more likely say that "Max is on good form at the moment", refering to their wit, whereas in great form might be more of a sporting reference as in "Arsenal are in great form at the moment".

It could be a cross pond-ism. I think that it is one of those expressions that is hard to pin down, the more I think about it, the more confused it gets.