You aren’t really pushing film with diafine the way you can with a normal developer. Diafine may give you a different speed than the rated ISO, but that’s pretty much the speed you get – time and temperature don’t have much effect on development*.
That said, you can get pretty decent results even with significant overexposure, especially with films like HP5 that have pretty wide exposure latitude anyway. I expect shooting HP5 at 400 wouldn’t pose a major problem, although you may lose a little detail in the highlights compared to shooting it at 800 (recommended speed).
(* I have seen some examples of pulled film, where the film was removed early from bath b, but I’ve never tried it. I believe you’d have to be much more precise about the time and temperature that the normal diafine process, and at that point you might as well get some d76 or some other standard developer…)