Here's how it goes. If a bike functions well, you can get a hell of a lot out of it. There are few things that are actually upgrades. There are some, but not a whole lot, and they're really not going to make you a hell of a lot faster.
In road racing, I've been seeing lots of entry-level racers with carbon rims, because they're now considered almost stock standard gear for racing... despite the fact that a lot of the riders sporting them aren't exactly being held back in field sprints by their gear. They're held back by the fact that they're not very good riders, yet. At higher levels, gear like that can maybe add 2 mph to your sprint (saving energy by being aerodynamic where it counts).
Much, much, much more crucial than better parts is an understanding of what is going to make you a better rider. Getting comfortable on the bike. Challenging your body. Working hard and recovering. Understanding how the bike works so that you can be a better bike handler, and understanding how your parts work so if something goes wrong you can fix it without breaking the bank.