I can't speak to the DJ applications, but with pro sound I've had experience both designing and building a number of cabs and using my system for 10 years or so.
First and in general, unless one is using rear/horn-loaded cabs, a 12" is generally considered a minimum/typical/ideal midrange or midbass.
Some companies (EAW, Mackie, JBL, etc) make cabs that are horn-loaded with 6", 8", or 10" drivers in the midranges down to the upper mid-bass regions, but this results in relatively large/heavy cabs and is done in order to get better projection, pattern control, and efficiency in the drivers' passband.
The overwhelmingly most popular mid-high pack is one or two beefey twelves with a good horn. The Eminence 12"'s would be a good choice, as would JBL's etc etc. But certainly the Emi's are a good value, particularly if one shops at some place like Image Communications. I've used a number of 12" & 15" Emi's and, like JBLs and some others, they seem to have pretty good voicing.
I've never really cracked the subject of (high/mid freq) horn design, it is a subject where there is even still a lot of compromises and differing opinions.
In general, there are a number of competing factors that must be played one against another in designing a compression-driven horn lense. Some of those factors include dispersion pattern and dispersion pattern as a function of frequency, trying to avoid having the horn display nasty resonances and other phase/freq-dependent problems, etc.
We've all experienced horns that DON'T do a good job of balancing those factors, typical characteristics include horn "squawk" and/or "nasaliness", and/or blistering SPL's (resonances) at particular frequencies or places in the dispersion pattern, and narrow or poorly covered patterns.
The market seems to have moved strongly to the constant-directivity designs. These are designs that try to make the dispersion more-or-less the same over the bulk of the passband. In my experience, constant-directivity horns tend to be some of the better sounding horns.
On the other hand, it might also be the case that the reason the constant directivity horns sound better is because a lot of the established companies are now using the design and they also coincidentally and in general do a better job of designing their horns.
As far as horn drivers go, I've had good luck with Emi's, and while not quite as good as JBL/R-H/McC/etc, they also don't carry the price tag. At least the Emi's avoid some of the bad/nasty characteristics that plague other lower-priced drivers, such as (again) squawkiness (resonances), poor voicing, that nasty metal "bite", etc etc.
A couple other words in passing. As Vance Dickason points out in his writings, even cone speakers (mid-basses and basses) have dispersion patterns in the upper reaches of their passbands. I don't recall the exact numbers and it varies with the size of the speaker, but 15"'s and to lesser extent 12"'s are already starting to become directional ("lobing") by around 1.5 kHz or so.
That is one reason that in general pro sound boxes tend to crossover at very low frequencies relative to, say, home hi-fi speakers. Most the better passively x-overed mid-highs/two-ways will be x-overed in the 1.2-1.6 kHz range.
The other reason for this is because the range of around 1kHz and on up to around 2.5kHz - 4kHz is *very* important to intelligibility and overall subjective sound quality. Using horns to cover that freq range allows one to better control the dispersion (audience/venue coverage) and the voicing (assuming one invests in good horns and drivers.)
I would not suggest the Emi coaxes for mid-highs, they are really intended for near-field monitoring. For example, I've had good luck using them in decent lower-cost stage monitors, particularly if one is talking about small-to-midsizish stages with lower stage volumes (such as small to mid-size churches, grange halls, etc.)
But the Emi coaxes are not appropriate for mid-high pack usage, at least in my estimation.
Good luck.