Unless you aero test before purchase, there is a good chance you would buy a slower bike given your criteria.
If you go all the way to a P5X there could be 2-3mins

Where you put your bottles, your helmet choice, and your clothing choices will make more of a difference than the bike. The bike frame is the most expensive place to buy very little speed. You might get a couple minutes out of a new frame compared to your old one, or maybe not.

Dam, sounds like I will have to be sensible and train.
Next someone's going to tell me running is important.
:0)

Running is overrated. It's all about the bike.
You could probably look for some upgrades to your existing kit (as ZTB mentioned) that would refresh your ride and give some benefit. Like new generation bars that allow you to tweak position more precisely as well as offering aero gains.

You suggest changing the frame and the wheels. Are hed3 wheels the ones which are about 45 mm deep? Than probably 90 mm deep wheels will help. And as other posters said, a frame upgrade from a p3 will probably cause a lesser change. (Altough you would only really know after exessive testing in a windtunnel which you probably will not do).

If you are generally thinking about buying a new bike with disk brakes (which are nice) just go for it.
Or are you ONLY looking to gain speed?

What I really was saying is...is it worth it if all I get out of it is a few minutes in an IM.

The P3c was considered as one of the first "super bike" frames, the wheels have only been marginally bettered and are still a gold standard in TT racing.
I wondered if, realistically, I was going to do better than 5 minutes with a new bike and disc brakes, which in today's world would amount to about a thousand bucks a minute.

I have too much Scottish blood in me to throw away good money that could be spent on better whisky.

Spend the money on a coach instead. I bet it buys you more speed than a new frameset.

I've got the P3C and have pretty much optimized it. If I had a grand or two burning a hole in my pocket and wanted to use it to get faster I'd probably book a session with Aerocoach at the Boardman tunnel and/or track.

You could get a bigger benefit from a new lid than a new frame, depending on how well the one you have works for you.

I think you'd feel better about buying a new bike to get something that has some cool new features that makes it worth it. My old bike was getting worn out, so I justified getting a new one by also getting electronic shifting on it. Sure, I could have put e-shifting on my old bike, but that bike is already close to retirement and electronic shifting is cheaper buying it on a new bike than buying it in pieces and then adding it. So I justified it by saying if I successfully completed a really hard race, I'd buy myself the new bike with electronic shifting as a reward and as my birthday present. E-shifting probably only saves a couple minutes because you are in a better gear more frequently, just like your frame might save you time, too.

In your case, I'd buy the new bike to get the fun factor of disc brake wheels (if you can also afford those new race wheels). A new bike every-so-many years keeps the athlete interested and happy. Testing shows you might go faster since you feel safer with the braking, too. Definitely so if you live someplace hilly and rainy.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

I have talked over the pros and cons with my wife and we have come to the conclusion that Granite Counters would be nice.
(sorry really old joke).