I have 2003 Specialized Allez Elite, that I do triathlons with . The problem is I want more speed I avg. 19.8 on my last tri. I have about $500.00 to play with or keep saving. The bike is all stock with aero bars. Some components 27 speed,105 all around,carbon forks and seat past.
Consider taking the 500 dollars and hiring a coach for 4-5 months. Seriously–I’m not being an ST troll. 
I think you’d see far better results than buying a set of aero wheels, for example.
Good luck!