My soon-to-be-started 96" S&S EVO build includes reading the instructions very carefully. It has the same warning as in the TC instructions Tank linked to - max 400F.
I do find it strange that the cases could be warped at such relatively low temperatures, but my guess is that someone have heated the cases to 400F and then done someting stupid during cooling - like put them in water. It is not only the max temp itself that matters, but the heating and cooling process that matters. If you control these (as in slow change of temps) I am sure you could go above 400F w/o damage.
I am considering powder coating, but never tried it - what temps are specified?