Never even bothered testing that one - the superheavy cavalry rely almost exclusively on their sky-high armour rating for survival, and anything that halves it tends to start causing casualties right fast. I had to fight some Merc Sughdians in a rebel stack once, and found that out the hard way.

Far as I can tell the idea is that the Clibs are what you take along against enemies that wield a lot of well-arnoured troops, while the Catas are intented for steamrolling lighter opponents - just about any other cavalry short of other superheavies would probably be among the intented targets, as would many types of line infantry. Kinda overkill probably and likely redunant, but at least it's nice to have the choice and variety. I tend to wield the two types side-by-side on general principles, and because I think the Catas look neater. Both pretty much squash anything they're pitted against anyway so the practical difference isn't that great.