I'd think, if wanting the firmest leather strop, I'd go shopping for a quality barber's strop, as the better ones actually process the leather (cased/compressed/hardened) for the best performance on razor edges. I don't have one, but the top-tier barber's strops in horsehide would be what I'd be looking for, IF wanting a high-performing bare-leather strop (used with NO compound). Otherwise, there are much simpler, more readily-available and much less expensive materials available for use with compound; I'd argue they'd even perform better than many or most cowhide leather strops used the same way. Any use of compound will render moot any characteristics of the leather itself, save for the basic requirement that it easily takes and holds the compound effectively; this is where some other substrates like linen/denim will do even better when affixed over a HARD backing. Leather of any kind really only holds a small advantage when used without compound on already-very-fine edges (like razors); some steels really respond to that, if capable of taking and holding such fine edges. Even then, expensive leather isn't really necessary, but might be beneficial IF the $$ went into the processing of quality leather (such as for first-rate barber's strops). I'm certain I'll never spend big $$ on another leather strop of any kind, EXCEPT in the possible scenario where I might go shopping for a barber's strop someday.
If firmness for strops is the absolute priority anyway, I'd be looking at wood instead of leather; I've liked basswood & maple for that. For both firmness and compound-holding ability, I've yet to see a better combination than linen-over-wood (firmly affixed with adhesive); the hard-backed linen's ability to take and firmly hold a lot of compound makes for extremely aggressive (FAST) burr-removal and polishing, with very, very crisp results at the apex. And hard-backed denim or linen used bare, without any compound, works very well for stripping a lot of fine burrs away.
David
