well, it depends on how good said player is.
91% of 50% shooting is .455
91% of 20% dreb% is 18.2%
91% of 60% shooting is .546
91% of 30% dreb% is 27.3%
(so, the better the player, the better off you are, but at the same time, the better player has more of a drop-off than the lesser one)
Personally, I wouldn't do it. 99% I am sort of okay with, but there's got to be a better alternative to spend your salary on.