The highest end cabs I've used are the SRX712M and QRX112, The QRX is big and heavy compared to the SRX and I believe either would put the ZLX to shame for performance with or without the horns coupled as you describe. The larger versions will give you more SPL but a 15 or double 12 are bound to be a little more muddy in the mids I think up to the point where the single 12 starts to distort enough to let the more robust cab take honors.
I like the idea of having both small and large cabs to perform a variety of functions at a wider variety of gigs. Personally, no matter how well done, even if it sounded OK, I think using the one inverted cab over another looks tacky. If you want to appreciate the higher end cabs, rent some first. You'll make less money on those gigs, but you'll learn which you like best. Also, when deciding on cabs, consider strongly what you can rent from another provider for when you have a gig where more, but same-model, cabs are needed. It will keep you from losing that gig to a larger provider.