I have inquired about them. According to them, on a ZHP you could expect 30hp increase with their stage 2 head and a set of slightly more aggressive cams (like the ones TMS used on their project ZHP).
I am desperately trying to find the funds to buy one of these stage 2 heads when my motor gets replaced. But with the cost of the core engine and the head I'd be well over $4000 total. May have to put it on my "LOOOOONG" term list of mods.