Caveat: I've heard a lot of heritage, but never HII & HIII side by side.
Per fellow Klipschies who share the disease, the HIII are the best of the Heresy line. Higher sensitivity (+3db over previous versions), better drivers all around, and better crossover networks. Since you're not a heritage virgin (you have Cornwalls w/ Crites mods, correct?), I think you would appreciate the improvements made with the HIII. Those same Klipschies typically rate the HII as the worst iteration. All of them will be limited with extension, there's only so much you can do with that sensitivity in that small-ish box.
Klipsch offers a HII to HIII conversion kit, so if you already have HII's (edit: I see you do), you can make them into HIII's relatively inexpensively. Bob Crites may have some tricks up his sleeve for your HII as well. He's got tons of experience with the old Heritage models, and happens to be a pleasure to deal with.