I kinda agree with you - it's a shame to goop up a beautiful ceiling like that. One of the interesting things about acoustics is that it's not a science we've learned a whole lot more about recently. We can put theory to the practice but the practice is largely settled. Whoever built that space knew what they were doing; they also knew that there were better places to put diffusion and absorption than the ceiling.

The acoustic lens you already have there is designed to break up flutter echo. It's already diffusing. It's no doubt designed to work with a carpet on the floor and, possibly, a bunch of books lining the walls. Do you have any illustrations or pictures of what the space looked like when originally built? I'ma guess that it was intended as a standing-room lecture hall, and "standing people" combined with that ceiling would have made for a pretty effective lecture space. I'm going to guess that the problems you're having are related to it being too empty, and too free of "dead cats" (the unofficial term for any acoustical component that are related to the presence of an audience).