I say, if you have a good fitting road bike stick with it for a while. Start by picking the low hanging fruit. What I mean is that you will get the most bang for your buck by changing the aerodynamic profile of the rider. You will be "almost" as fast as a dedicated Triathlon bike. When I upgraded my road bike for Triathlons I made changes as follows:

1. Added Stubby triathlon bars!! Big difference. They are designed to go on a road bike and will help you maintain your riding geometry. MOST bang for your buck!!

2. Removed frame bottle cages and added "between" the triathlon bar water bottle.

3. Got tight fitting Triathlon clothes.

4. Got aerohelmet

5. Added rear wheel AeroJacket covers.

With these additions, a good aero fitting and time in the saddle to get you comfortable staying in a aero position for a extended period of time you WILL be faster. How much is based on your cycling endurance.

After I made theses changes, I was averaging 3-4 mph higher speed for my normal training loop.


PS:
I used this setup for one Triathlon season. I eventually did get a Triathlon bike. I was faster on the Triathlon bike, but it was pretty expensive for the small increase in speed I gained!