Backwards reasoning. Others already said why. What shooting HP5 at 800 does is simply shove the shadows even further down a long sweeping toe, creating more undiscernible mush down there, rather than improving crispness. You're just lopping off low value information, not improving it.
With HP5, I try not to shoot it in high contrast scenes; but do know how, if necessary, to apply unsharp masking to get the most out of that kind of situation. I rate it at 400 and process it in PMK pyro, but never shoot it in any format smaller than 8x10. In other words, my own interest in this film is not for sake of street shooting or photojournalism, but for sake of rich prints.
If people are afraid of pyro, that's fine. I have some more liquid A&B concentrate of PMK being shipped right now from Formulary. No need to fool around with powder. And nitrile gloves are routine in all my sink room activities. Most people have more dangerous chemicals under the average kitchen and bathroom sink, or out in the garage. Do you think all those home use herbicides and insecticides are safe? At least we don't go around spraying pyro.
Andrew, although I haven't shot HP5 for awhile, one of my preferred tricks was to go ahead and expose it at 400 (not 800 !!), and then counterintuitively overdevelop it, processing it in staining PMK. That expanded the midtone gradation wonderfully, and optimized the lovely almost etched edge effect this film is capable of. But it also created a very dense negative hard to print, especially with respect to the highlights. That's where the unsharp mask came in. It allowed me to have my cake and eat it too. Wonderful prints.
But shooting TMY400 instead is just so much easier.