Originally Posted by TMonk

You already know this, but

1) position on the bike
2) power
3) aero bits

1) and 2) are somewhat equal in importance with 3) coming much farther behind.

Having said that, the sum of all incremental equipment gains does make a difference. My preferred training setup is a Giant TCR regular road frame, regular classic bend drop bars, lightweight vented helmet, box section wheels and mid-grade tires. When racing or simulating race efforts, I ride a Specialized Venge (aero frame), Bonty "aero" road drop bar, a Spescialized Evade aero helmet, 50 mm carbon wheels and fast tires. The second setup altogether is a bit faster than the first, uncomfortable as though it may be.

In San Diego we have Fiesta Island which is a 4.25 mile one-way fast loop, a perfect playground to experiment with this kinda stuff. A hot 300w lap with the first setup for me is typically around 10 minutes with setup one, about 9:45 with setup 2.