Interesting question. I have a hybrid that I jut used from my first tri. While I averaged 16 mph, I got tired of all those on road bikes zooming past me. So I am considering a road bike for my next tri, but have budget constraints. One of my options is getting a Specialized Allez at 699 (lower end; sport is next model up), and upgrading it over time, and my other option is waiting longer and purchase a better bike at 1500.

My Upgrade Analysis:
Assuming the Allez you have compares to the ones out there now your components are Shimano Tiagra/Soro. your next level up in components (whole group sets) is Shimano 105 ($500+or-), Ultegra ($1000+), Dura Ace (don't ask). Alternatively you can go down the SRAM path with SRAM Rival ($900 + or -) or SRAM Force ($1300 + or -). And there are wheels. I personally like the ones that have the additional one or two inch rim on the inside (Bontrager Aeolus at $750 and up per wheel).

Purchase Analysis:
At 1,000 to $1,300 you can several nice bikes with Shimano 105 components, but the frames are going to be similar to what you have. Aluminum with a carbon fork. Felt F85 ($999 but the shifters are by MicroShift), Trek 2.1 ($1,300), Giant Defy 1 ($1,300). Up your budget to $1,400 Fuji has a Carbon Fiber Bike at Performance Bicycle the SL 1. Buy the time you hit $1,700 there are many entry composite bikes you can consider with alt least a Shimano 105 component set or similar. But it seems you get what you pay for. For example Felt's F85 at $999 is a bargain, until you realize that shifters are a pricey part of the component group set and they are using the cheaper MicroShift (not saying they are not good). Most reviews on new mentions the wheels could have been better on the models that are priced lower.

Conclusion:
I still have a dilemma because I don't have a bike frame. Your choice is simpler. Composite frame or not. If not, the Allez frame is a good frame to build on. You will end up paying more than $1,500 for your upgrades over time, but your bike will be unique, and probably better than any Allez out there. If you are fine with the Aluminun, go for the upgrade.