I've got an article in a book up in the loft somewhere about playing with XP2. Who was it written by.... Hmmm, the name Micheal Maunder springs to mind, but I'm not sure. Was he the Celler Steller guy? He may be here on APUG somewhere...
Anyway. Point is that in C41 processing you have a very, very weak silver BW developer to give a matrix for the coloured dyes to form on. This is got rid of later in the bleach fix.
So... use a conventional BW developer and you get a strong silver image, perhaps not so far from HP5+. Except, the colour couplers are still intact. So, say you push the film as hard as you can in really hot print developer or D-19 or something. A C-41 film will stand a bit of heat & extremes of pH if you want to stew it in something really alkaline and so you already have a film that can be abusively pushed really well - but then, you can activate the colour dye as well by dunking in C41 to add even more to the density (but don't blix it, obviously). You can loop it too, to squeeze out even more. The idea is you can get really over the top film speeds ('speeds', if you prefer ;-) - really over the top grain, too. I think the author was talking about effective EI of 100,000 or something silly? I think the intended application was for astro photography, so the contrast may have gone beyond the very useable for more conventional photography.
I really must go and find the article, I haven't read it for years and I wouldn't trust my memory too much
Anyway, there did seem to be much fun to be had with this film for those wishing to break the rules.