I just want to say you have a really cool looking bike.
That said my current bike went 38 mph on a downhill 2 weeks ago during a Triathlon. I do have aerobars which puts me in a more aero position, but otherwise I have the identical setup to you (albet I'm probably a little larger, 155lbs on a 54cm bike). Firstly to hit that kind of speed you need the right hill
hehe. As well you need to accelerate to that speed, you cant just drop to the 12 rear and up to the 52 front and expect to hit 35 mph, you will kill your legs, you need to step through the gears as you accelerate and get your cadence up.
My opinion, keep what you have, but a wireless computer with a cadence meter, practice holding 100 rpm as you ride, you will find out a lot about what your current gears can do. Most downhills that arent that steep I only hit 30, if I want to go any faster I really need a long hill thats a little steep. You can go faster you just need to practice it right.
Also, if you are going to spend the money to upgrade, get yourself a Shimano 105 setup, you can get it fairly reasonably priced if you search, its stronger lighter stuff, and 10 speed rear as well, with a triple front crank that gives you a lot more gear options. Once you get used to the setup you have now you will see why you would want something like that, and its less about top speed and more about going faster overall.