Upgrade to Tri Bike?

Perfect!

OP, you thought about borrowing/loaning to try out the aero position and twitchiness of a tri bike?

I went through the same thoughts. Is spending a few thousand worth x amount of minutes in racing?  I'm glad I made the investement and bought a carbon tri bike.  Besides the benefit of going faster, I apprehiate the riding experience better.  I totally baby my new carbon bike because its worth allot more then my old road bike.  In addition, I found I can go 1-2 mph faster on my Tri bike with a quicker pick up.  I compare this feeling like driving a Honda vs a Porsche.   It was wierd riding it the first time and how the air just flows around you.  However, when I made the switch, I had to relearn how to ride on this bike, took me many weeks to get comfortable riding in the aeros.  So..outside of maybe 15 minute faster bike times; will you train better, train more, love to ride the bike more, and have more fun...if the answer is no to these questions, then the investment is not worth it.  I personally love just riding mine, specialized transition pro.  Honda vs Porsche..which would be fun to drive, both will get you from A to B.  

If your looking for value, a P1 or a S32 will get you most of the savings a bike 3X as expensive will get you, everything after those will be marginal in terms of position/speed/comfort.  Get a used one for under a grand and get some decent wheels and good lid and for ~2k you will be at 95% of your equipment max in terms of value.  Another 2-20k and you can get that last 5%.

Yes a tri bike can make a fairly big difference. Almost all of the advantage comes from you being in a more aerodynamic position. This is exactly why you should book a bike fitting FIRST and THEN look for a bike that fits you, instead of vice versa. $2500-4000 is a fair bit of money, so I do assume that you want the best bike for that money and the answer is the one that fits YOU. You can be a whole lot faster on a well fitting $1000 bike than you can on an ill-fitting $4000 bike...

audiojan raised a very excellent point.  I got beat by people on way less expensive bikes in my last Tri, first time with my carbon Tri bike, because the fit was bad.  Felt like I had no power and hard to breathe.  I got refitted by another person and he raised my seat up almost 5 inches..talk about a night to day experience.  I kept wondering why I was being passed..thought it was a fitness issue.  What I noticed was that I caught people on the down hills because of the aero benefits of my bike but lost them on the uphills and flats.

I have nothing to add really...just GO FALCONS!

I'm done.

Okay, I'm not.  If you enjoy doing tri's then having a tri specific bike would definitely be worth the investment.  The BEST thing you can do is buy a bike that FITS PROPERLY if you want to make it a long term investment.  There are a million brands out there so just go ride some bikes.  There are some fabulous stores in Colorado.  Most will let you try out bikes.  Just tell them what you are interested in and they can point you in the right direction.  The BEST way would be to figure out based on stack and reach what bikes *should* fit you.  That narrows down the list a whole lot right off the bat.

I Went from a 1300 dollar Allez Elite to my P2, and Ill never turn back. Now the P2 just dropped 400 dollars for me it is steal at that price.   I'm not sure how much of it is just a fitment thing, but I can ride my tri bike all day long.  I went from struggling to maintain 19mph on my road bike to easily in the 21-23mph range over my regular 30 mile training ride.  It was worth for me just due to the comfort.  Oh, it also looks a million times cooler than my road bike ever could. 

Morpheus, I saw something similar with me going from road to tri bike. Even on the trainer, I can tell just by bike speed that I increase speed just being on the aero bars then on the hoods, that is without applying any extra effort.  I think maybe the leg weight might help more with the downward push of the pedal on Tri bikes.  This might be because of the angle to the pedal is different in road bikes to tri bikes.  Also, I found I didn't need to shift as much to get to 20mph on my Tri bike, maybe one gear. My road bike, I had to keep shifting and build up the speed..while staying in the seat. I felt the difference was like "spinning up to speed (road) vs push forward with speed (tri)" Lastly, my last group ride on flat/long gradual hills, I felt I had to ride slower and different to ride a slower pace with the guy next to me.  The guy next to me was smooth circle pedalling and I had to pedal/coast allot just to keep next to him and not fly past him.  I would love to see a power watts output comparison between same biker but different bikes: road vs Tri bike on trainer or same test condition.

Get a tri bike
Train Hard
Crush Dreams
Make people think that your bike made you that fast

You can buy a used P2sl (or the equivalent from whatever brand fits you) for about $500-$1000. Any money you spend past that is "look at me" money.

Is spending money on a bike to make you marginally faster worth it? It is to me, it was to a lot of people... whether it is to you, only you can answer.