I have no experience shooting animals heavier than 15lbs with the .221FB and my preferred 40gr bullets. If I intended to shoot 80lb springbuck I would expect that a heavier bullet would perform better.
The 52gr A-Max might be my first choice. Heavier bullets from the tiny .221 have very low muzzle velocity. The Barnes X bullets would give reliable penetration but the accuracy is HARD to find. I tried several different combos in my custom rifle
and eventually gave up.