Reducing the spread seems kind of irrelevant. You make a solid wall of purifiers, or the corruption gets through. The rate of corruption growth doesn't factor into it. The only thing is, if you have enough people and you're fast on a new round, slower growth might let you win the round more quickly by saving the existing uncorrupted space. But even then, if you spend time killing hearts, you give up time you could have spent walling off the uncorrupted space.
That said, it's not entirely clear the rules by which the corruption spreads. Have you played any of the Creeper World games? I'm not saying you should copy them, but it's an example of a reasonably elaborate ruleset leading to reasonably varied player choices, with a similarly amorphous enemy.