Fit is important. When my fit is right, I can climb like Hans Florine, have a cadence like a centrifuge, and descend like a falcon. These are all exaggerations, but they sound good, no?

Seriously though, I think that fit is one of the most important upgrades you can make. I agree with CDR, but would like to expound that usually fit doesn't make you faster, so much as it makes you more efficient (which is faster). As an example, if I'm taking a long solo into a fierce headwind and my fit is right, I can churn along for several miles in the IAB or drops position, and for the most part, all my muscles have to focus on is what it takes to keep my upper body still while my lower body works toward steady locomotion. If I were to have my saddle height changed by a few millimeters, and do the same run in the same conditions, I would be more fatigued in few hundred yards. The reason for this is simple, the fewer muscles that are working at stabilizing your body, keeping you aloft, and keeping your joints in alignment, the more of them can go into moving your legs. The more efficiently you propel yourself, the longer you can do it at a given speed/caloric intake.