I would expect a good specialized centre channel speaker to be far superior than a third main. The centre channel in an HT setup is primarily for voices. Voices are in the midrange. So the design of the centre speaker can focus on midrange reproduction. There is less need to design for bass. The left and right mains, on the other hand, need better bass capability. So their design will be obviously different. Specialization of function is, I think, the primary reason why centre speaker design differ from that of other speakers. Also, HT speaker design will, or may, at the manufacturer's option, have to accommodate the design requirements needed for THX certification.
You can't assume that what's desired in a speaker for music reproduction in a 2 channel system is what's needed or desired in an HT set-up. That's why a lot of people have dedicated systems for both 2-channel and HT use. That's why you see so many posts on this forum from people about how to deal with the compromises when they want to use a single system for both purposes.
