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I have no idea I'm afraid! I've never owned a Rapha Jersey.
I only had a chance to get a 40km ride in today so not exactly an exhaustive test but with a long sleeve jersey underneath it was a pretty good temperature. It seems fairly breathable so even when I was putting a bit more effort in I didn't get too much of a sweat on, not as boil in the bag as you might think.
I think with a short sleeve jersey underneath it'd be fine a few degrees warmer and with a midlayer you'd be fine a fair few degrees colder. Will report back when I've had a chance to test it out a bit more!
How is it different from the Brevet Long Sleeve Windblock Jersey? Because I have that and it's been bang on in 8. The jacket is a bit sweaty looking.