I wouldn't think a bushhog would have any effect on the HST as far as causing overheating due to the ease of pulling it. Ground engaging equipment like disc or subsoilers could overload it but as long as you get the right gear for the work and don't stall or semi-stall it , it should work all day long without a problem. Just watch the radiator and keep it cleaned, that is what will be your issue with cutting tall grass and weeds. It may not take very long either to stop up your screens so keep a close watch on the water temp. gauge and if it starts to climb toward the red, stop and clean the radiator screens. You may have to take compressed air/water hose and back blow the radiator to get some of the chaff out that will for sure slip thru the screen.
15 acres is not a large area and likely not more than 3-4 hours work at most so if your tractor cant handle 4 hours of light duty work, get rid of it and get a real workhorse tractor.
Edit PS: Do you really think you need a 10 foot cutter to cut 15 acres of pasture. We cut 40 acres with a 6 foot cutter. I could not justify spending that kind of money to save an hour of cutting time on only 15 acres and your proposed 4720 tractor would be on the edge of being underpowered with an 8 foot cutter.
