As far as photosynthesis goes, there is such a thing as too much light. I think it's called excitation quenching or something (from memory) where excess photons are converted to heat which changes the chemistry of those processes which make up photosynthesis. I don't know too much about it, but apparently it's possible.
I don't know how that links with calcification, but remember that zooxanthellae and the corals themselves are competing for alkalinity.
All in all, I don't think you'll find a lot of info on this. There isn't a lot of consistent info around on the links between zooxanthellae, photosynthesis, and calcification. For every study you find saying one thing, there's another which says the opposite.
My gut feeling, and opinion, is that 400W halides are overkill.
Also could what you're seeing be a result of changing bulb brands? Some bulbs, as bright as they may be, are crap for growth, others are excellent, all depends on PSD's
Layton