An increase of 8 horsepower when you are over 80 hp isn't as much as you would think. I own a JD2555 and a JD2555 that had a 2755 engine installed in it. From the operating station, I can't tell the difference in the two tractors doing the same jobs, such as pulling the 15' batwing. There are differences in the two tractors, but they arise from one having a loader and 4wd and the other just being 2wd.
I am thinking that what you do with that 81 horsepower versus the 89 horsepower is going to be the answer to your question. There are some operations that use a lot of hp and some that don't. Whatever it is that you think you are going to improve with 8 additional horsepower, probably won't show up from the operator's station. It might show up over the long term in a spread sheet, but I doubt it.
As much as I hate to say it, I doubt spending the money and time will be worth it in the long haul.
Another example when 8 horsepower makes a difference is on smaller tractors. I went from a Ford 1100 to a Ford 1210 to gain 3 horsepower. That was just enough to allow me to pull a 5 foot finish mower. In reality, I wasn't really satisfied with the 1210's increase in horsepower and when I acquired a 29 horsepower tractor, it became my 5 foot finish mower power source.
David from jax