It's possible I've got my 2s and 3s mixed up. I think Bussell's been on a P2 this season, and it's not like he'd be riding a bike that gives much aero penalty...
The whole question is made worse by Cervelo's naming convention where they will happily replace existing models with completely redesigned new models but use the same names.
I ride an old style P2c, the same as Chrissy Wellington did prior to switching to Cannondale. I love mine, but don't think they were ever considered faster than the famous old style P3, although the geometry did work better for some people.
Despite it's age, the classic old style P3 is the frame that I would say has very well regarded aerodynamics. However nowadays I think that is more to do with the long and low (3cm compliant) geometry than the tube shape. Also these frames were designed with frame aerodynamics as a priority and so aren't particularly well optimised for carrying multiple bottles etc. A bad thing for triathletes, but pretty irrelevant for most testers.
In 2013(?) Cervelo introduced the new style P2 and P3. The frame shapes on these are undoubtedly more aerodynamic than the pre-2013 frames, however Cervelo also targeted the new frames at triathletes and therefore followed the trend of high stack and short reach, as well as improving the integration of bottles and bento boxes etc.. While this might benefit Ironman athletes it doesn't make them ideal for CTT riders. Bussell rides one of these new style P2s so Xav has clearly found a way to make it work but many Testers will struggle to achieve an aero position while remaining 3cm compliant. The interesting thing to note is that the frame shape of the new P2 and the new P3 are identical. The only differences are in the weight (different carbon layup) and a tiny adjustment to the fork which is again apparently aero neutral but lighter on the p3. Given that there is no difference between the aerodynamics of the frame I have no idea why anyone would spend £££ on a few grams, and clearly Xav agrees.
I think both the P4 and P5 are considered very fast, but again suffer from being designed for triathletes (especially the p5). Finally Cervelo have made the final step towards integration by designing an Audax bike.