I have a Trek with 105 11 speed and a P2C Ultegra 10 speed. My Cervelo is carbon, so it's not exactly the same situation, but my Cervelo is faster than my Trek with clip ons. For training I use the Trek mostly. On rides with steep climbs (+10%) the 11 speed is easier for me (but not overall faster on 50mile+ rides).

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Not a coach. Not a FOP Tri/swimmer/biker/runner. Barely a MOP AGer.
But I'm learning and making progress.

For 400 I'd say go for it. A dedicated tt or tri bike is faster than a road with clip ons. More comfortable geometry too for tri (could get special seatpost for roadie to steepen seat angle but still, I say go for the P1). Plus, for a full distance race, you will want to be able to shift from the extensions. I raced sprints on a road bike with clip ons when I started the sport, and getting my first tri bike idk how I ever tolerated moving my hands from extension to hood every time I wanted to shift.

If the gear ratio isn't enough, maybe there are some different cassettes you can get, probably easy to find on eBay. Or heck, of you love the bike I'm sure you could get 11 speed 105 5800 or 7000 parts for a couple hundred bucks.

this new user is apparently a scammer. if i'm wrong, i'll revise this. just, beware.

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Dan Empfield
aka Slowman

I'm still riding a 2009 Cervelo P1. I've changed the 12-25 cassettes to a 11-28, and I've replaced the front 50-34 front rings with 52-36, for better top speed. I've also got Flo 90s on it. Its a good old bike, and I'm still winning my age group in all my local tris. It's beat a lot of carbon, and I have no intention of replacing it anytime soon.

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Athlinks / Strava

I belive the P1 was the same bike as the P2 in aluminum (P2K and/or P2-SL) once they started making the P2 in carbon.

It's a great bike. I've had a P2K and still have my P2-SL, which is now a full time Zwift/trainer bike. Not a lot of difference between the al and the carbon, in geometry, handling, or weight, just a little different "road feel," between the two.

I'd have no problem doing a full IM on that bike. Other than a bit more vibration from the aluminum, which you can mitigate quite a bit with the right saddle, bars, wheels, and position, it'll hold its own against pretty much anything else on the racks, as long as you put the work in.

And Ultegra 10 is a great group. Really no need to change anything there. I agree with the above, too, that an 11-28 will pretty much work on any course, although a 12-25 is pretty good as well.