I had a William Evans paradox built for the Westley load at 1550 fps and that was in a 8&1/4 lb gun......recoil was brutal. I had it out at the range one day and three of us shot my paradox gun and a friends Army & Navy .577 3" double, everyone agreed the .577 was more manageable in the recoil department. I am not so sure that the early black powder H&H paradox guns were heavier than your gun. I do agree that 900 fps should handle Whitetail just fine.
Thanks for the reply
I compared a black powder load, with a smokeless load, and found the smokeless load to be much more agreeable.