If you're setup with temperature controls for C41 processing, why would you try room temperature other than as an experiment?
I'm interested because the Minox daylight loading tank doesn't lend itself well to water bath to keep the temperature at 100F, and if it works well at room temperature I can use the Minox tank without all the setup to fill and warm a water bath, warm the C-41 chemistry, etc. Not to mention the temperature and humidity levels in my darkroom when my
sous vide has been holding a water bath at 100F for half or three quarters of an hour. Plus, the tiny tank and a powder chemical kit are practical to haul along on a vacation trip (doesn't even need a dark bag if I don't need to reload cassettes), where my
sous vide and accompanying paraphernalia are not, even if I'm driving vs. flying.
It's been my understanding that in order to produce black image density all three color couplers (or an even broader range) need to be present in each emulsion layer of XP-2 Super, so I wouldn't expect any changes in spectral sensitivity -- unless I've misunderstood how B&W chromogenic films work in general. In fact, I see no sensible reason why the film would have more than one emulsion layer -- and the "money" argument suggests it ought to have just one, because unless it's coated on the same line as conventional color products there's no reason to have three (or more) emulsion layer coating steps.
FWIW, I have attempted the room temperature stand development from the Lomography article -- I didn't see the results as horrible, but it there was no good reason to do it that way when I was using my Flexicolor chemicals and Paterson tank anyway. I won't be using stand in this case, however. I'd be interested in the Digibase figures, though.