I have no personal experience with this combination but I'll lend you my thoughts!
I think you are cutting it to close with the horsepower. Sometimes horsepower is used to justify the tractor size but with this being a PTO machine and not weight issue then I'd say you have no reserve. You might find someone else who has pushed this mower before with less horsepower that might answer you with more information but knowing the marketing of many companies if you could run this machine with less horsepower they would have stated that.