I wonder whether keeping the names of data constructors in info tables is expensive?

I would have guessed not. (more debugging info such as function names and src locations is likely to be more expensive however).

Anyway, would it be possible/feasible for GHC to keep names in info tables all the time, not just for profiling? At least for data-constructors?

That's a distinct possibility. If you send patches we'll incorporate them. Cheers, Simon