I agree, I don't think the slimline will give you enough coverage for a 24x24 cube. It doesn't look like it has good enough optics to disburse the light. That model looks like it would only would work for a nano or something. My cube is being built and it's a 25x25x12 and should be done in two weeks btw.
You mention the slimline S, is Tim going to be sending you this model as well to test/show for him? I'd still like to see the set up here sooner or later.