Sneezer--
I have run both the ZAC (Kawahara) foams and the Mugen Fast foams.
The ZAC foams seem to make my car (Mugen MTX2) a little more stable vs the same set up with Fast foams (same shore rating for both sets --45 fronts and 40 rears, 26mm all around).
The difference wasn't monumental, but I did notice a slight difference in terms of stability. I'm certainly not a tire / foam expert, but maybe the multiring construction of the ZAC provides an additional degree of stability. The ZACs cost a little more, so whether or not the slight difference in feel is worth the few extra bucks is something that only you can answer. Overall, I don't think lap times were appreciably better with one set vs the other.
The hex hubs on the ZAC foams are really beefy and strong. I run a Richey RTU in my car and I have yet to strip out a hub. I have bent the heck out of the rim, but that goes for any brand of tire (front oneway + stacked 2x4s = bent rims -- or worse).
Hope this helps,
DT