I have a lot of HYdroton left too. In fact, I just got a 10 liter bag before I read about this Higromite. It's cheapo though, Higromite is like $22 US for 20 Liters. I will pick some up for the next grow and try it. It will be tough to compare the two though, I don't use clones or identical plants. My grapefruits are 3 different sizes in the Hydroton due to 65% stability rate (65% similar in size) as noted by the seed company's description.
Like I said though I will be trying the Silica Blast when it arrives (by wednesday) because of the benefits it claims to have. So essentially I will be supplementing the hydroton with silica, which the Higromite claims to contain naturally. I wonder if Higromite and Silica Blast would be silica overkill? I thing the SIlica Blast with Hydroton will be a good combo. I'll keep you posted.