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I'm no huge RPR fanboy but I've seen the way they train and race for the last few years (coincidentally I was at one of Arlen's first cat 4 races - he was ludicrously strong). The intensity at which they ride is perfectly suited to making large gains very quickly provided you can keep up the workload. Tom Percival's blog is a good insight into just how much work some of those guys are doing.
I think he may still have the fastest known Regent's solo lap, something like 5.35.
Edit: Bit of fact-checking reveals RPR riders have gone faster than this since then, impressive!