In stock form, a few tips for some slightly improved times:

• Perform a throttle reset (search for it, tons of threads cover it)
• Race with top up... less turbulence
• Wing won't matter... it's only there to spoil the air, it provides minimal drag
• Make sure you're running with clean air filters (you'd be surprised to see how clogged they can get)
• At the staging lane, bump the autostick into manual mode. You should be in first gear. Hold the brake and give it some gas, loading the rear wheels. Only rev to ~2,000 RPM... Watch the tree carefully, and on the last yellow, let off the brake and jam the gas. Don't worry with trying to shift; the autostick will do the work for you, shifting at redline. Leaving the stick in "D" causes your car to shift early, killing your times. Last but not least, make sure you're racing in "S" mode, not "W" (this is the small switch beside your shifter... press down on the "S" end). W mode is winter mode... you'll launch in 2nd gear.

You should be able to run a mid/low 15 simply by doing those things... Lastly, practice practice practice!

Cheers,