The Beta 10 won't handle extreme pressures, but up to 100 watts or above 80 Hz it's perfectly comfortable; I've been using them in horns for years with no ill effect. I've had no problem with them structurally up to 200 watts below 80 Hz, but by then the power compression causes so much throat distortion that you're not going to put that much power into it for very long.As long as the specs are OK you could certainly use premium drivers in my boxes. I don't personally anymore, as I've found mid priced drivers to be adequate, and saving money on drivers is one of the nice aspects of horns versus direct radiators.
The Titanic III (it is an Eminence OEM for Parts Express)is actually very similar to the HL10a, but with its lower Fs and higher Qts it isn't as well suited to horn loading.