If there's one bolt-on upgrade that will definitely cause measurable speed gains, it's an aero bar. At cruising speeds on level ground, I typically get 1-2mph boost when I'm on the aero bar. There are many caveats, however: can't use them in mass-start races, they may be viewed as a below-the-belt way of getting faster since there's no training involved, and since they put your hands out of easy reach to your brake/shift levers, they're far from ideal for riding in traffic.