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1-3 mph quicker
As much as you probably want to justify spending 1.5k on wheels (and you don't need to as they are very nice!) There is no way on earth that 303s give you 3mph difference in speed over a standard set of wheels. I reckon 1.5 mph tops. I don't even get 3mph extra going with a disk wheel and 80mm front on my tt bike.
Not convinced about the new 303s though..72psi max seems a bit limiting, not sure you could drop narrower than a 28 with that pressure limitation (i prefer 25s)
I have my eye on parcours wheels. I already have a set and like then, but thing that puts me off another set is how hard it was to fit tyres. gp5000 were pretty much impossible.