The only issue would be a Kirk injury, as mentioned we wouldn't want to run with Heineman/Sanchez for an extended period.
If you are getting 100-120 games from Kirk, the "upgrade" over Heineman for the remaining 40-60+ games might not be worth the cost.
For what it's worth, Zips projects Heineman at 0.9 WAR in 63 games, and Grandal at 1.5 WAR in 88 games.
Steamer projects Heineman at 0.5 WAR in 33 games, and Grandal 0.8 in 41 games.