That will not help, because zombie hordes are not the result of natural spawn mechanics.
Zombies simply have a chance to spawn an additional zombie when you're not directly looking at them. Every zombie spawned in this manner has its chance to spawn additional zombies reduced by a small value, so eventually (after a couple of "generations") the horde stops replicating itself. Unless, of course, you pick up a new naturally spawned zombie while running away, that one can spawn its own horde.
I think it's neat, actually. 1.6 was the first time I died to a zombie since, oh, sometime in 1.2. And I do play on Hard difficulty. Making zombies a credible threat was sorely needed, especially after other vanilla mobs got buffed as well (creepers in 1.4, skeletons in 1.5). And honestly, the horde mechanic is way more creative and interesting (and way more threatening) than simply doubling the damage output or HP of a creature that can never touch you in the first place because it is too obvious, too slow and doesn't have any trick up its sleeve.
Well now it does. How are your zombie apocalypse survival skills?