You’re getting a nice bike, a more upgradable bike but initially you aren’t gaining that much. The difference between a 2005 aero frame and a new one is less than 10w. Frames have nearly topped out on aerodynamics. Where you get the biggest gains with newer bikes is electronic shifting, fit customization, 11/12 speeds and now disc brakes. If it was me and I didn’t upgrade very often I would wait and save for a di2 bike as that upgrade is significant.

What's your goal? Is shaving a minute or two off your 70.3 time worth ~$3k?

I'd ballpark that the new bikes are maybe ~10-20w faster. The additional adjustability might be nice. Better fuel storage. Ability to fit wider rims/tires. Integrated cabling + brakes looks better but can be a pain in the ass to work on.

Don't just upgrade because you feel obligated. Frankly, if I was gonna drop $3k on my first new ride in a decade, I wouldn't be going with a 105 level setup.

open question to think about - CAN you swap across the HEDs ?

Are the dropouts on the IA still traditional roadie QRs or have they changed to through axles ?
Are the HED Wheels' hubs convertable from what I assume must be 'traditional' quick release on the old bike to suit a bolt-thro new one ?

I've been in the process of getting a new bike and lots of stuff on the old bike is not compatible with the new one and vice versa-
- rim brake wheels with no disc mounting
- many newer wheels are wider do don't fit older bike due to no clearance
- change in hub/axle from QR to bolt through.
- Change in the number of gears (inc. 11 speed cassette doesn't fit early 10 speed hubs without some machining)
- BB / crank incompatiblies (threaded cs press fit, 24 or 30mm, etc)

Your HED wheels probably can't be upgraded to 11 speed.

I have a Felt IA16, and I love it. It is a fantastic all-around bike that is very fast.

Jumping to a modern bike would be a lot faster than what you have now. But, hard to say how much faster. These are some of the incremental advances over the years from yours to the Felt IA...

1. Cervelo made some pretty substantial aero improvements in the frame shape from your aluminum frame to their 2013-era carbon frames
2. Felt improved on the speed of those 2013 P2/P3 designs
3. The Felt cockpit is probably faster than whatever you have on the P3.
4. The P2/P3 line has always had dirty cable routing that puts a lot of draggy cable in the wind.
5. The rear mounted brake on theP2/P3 is not optimized for aero.
6. You can run wider tires on a current bike that will give you lower rolling resistance and a more comfortable ride.
   

All those little things can add up to a lot of speed.

The one possible downside, as others noted, is that your HED wheels may not be compatible with an 11-speed drivetrain. But, if not, that is OK. Think about selling those too for a wider set of modern HED wheels that are wide, crazy fast, and awesome .

You might be able to upgrade your HED wheels to 11 speed if they aren't already 11 speed compatible (depending on which build year it is).

Check out the HED site for the upgrade options. https://hedcycling.com/11-speed-conversion/

I've upgraded a couple of HED Jet wheels to 11 speed before and it's a simple swap. Only tools required are 2x 5mm hex wrenches.

Edit: Didn't realize you have a powertap hub when posting. Check with them but you might be able to upgrade that to 11 speed but I don't know if it requires a new calibration or not.

I had an old p2sl (2008) until I got a new Scott plasma frame last year. I really enjoy working on bikes. I gradually upgraded components on my old cervelo and other gear. A new frame/bike is a pretty minimal improvement in aero. Check out the book faster is you want a great resource:

https://www.amazon.com/...1937715027/ref=nodl_

I ultimately upgraded as a personal reward after lots of hard work and for the fun of the project and build.

You may gain 10watts in aero. You may gain some placebo effect. It’s ok to reward yourself and have fun too!