I admit that the ork reproductive speed is being overestimated- the 40k Wiki uses the word "Decades."
I wonder how the Orky Fungus-Creep would interact with the Zergy Fungus-Creep?
I still feel like the Orks are the closest thing to a custom-designed anti-zerg race that you're going to get. Rokkits, stik-bombs, flamers, and lots of dakka are going to work to thin the horde pretty effectively, and they enjoy melee enough to balance out enemy melee and keep their AoE units safe.
I mean, Killa Kans are 12 foot tall humanoid robots that have circular saws for hands and can mount a rocket launcher/flamethrower/heavy shooter. It isn't going to get much more anti-zerg than that.
The orks would get ripped to bits by the Protoss though.