If you do the sums:
Work to raise 1 kilo 20 km = F*d = 10*20000 = 20,000 (here F = force due to gravity)
Work to speed 1 kilo to 6km/s = 0.5*m*v^2 = 0.5*1*6000^2 = 0.5*60000^2 = 18,000,000
So you don't save much energy in terms of getting stuff to orbit (caveat maths might be wrong - I'm no physicist). But if you gain any advantage it's that gun-launches becomes more attractive. The main advantage of that is that gun launch is cheap and simple. Or at least cheaper and simpler than throwing away a rocket engine every time you send something into space. You get this advantage because the atmosphere is thinner the higher you go up. Air resistance is proportional to the volume of air you have to displace (think mythbusters shooting bullets into the swimming pool) so less air density means less drag. I have a feeling though, that at 20km altitude it's not going to make too much difference.
One disdvantage of gun launch is the G force the load suffers. But that'd be fine for a lot of goods: water, building materials, electronics (some modern artillery shells have electronics in them).
If you want to use gun launch you probably have to go higher. Which begs the question why do you need to build a tower? Why not have a blimp with a gun rather than a 17km tower? Ballons can get to around 50km. Space starts at about 100km (the point where drag is near negliable). The problem with this approach is distributing the shock through the balloon without ripping it to shreads.
Finally.. you can't get orbital insertion without a rocket to make your orbit more circular. So you either fire your bullets into a net in space, or you attach a rocket engine to each one. If the later, the rocket engine has to be heavier than usual because it has to be reinforced to stand the launch g-forces.
So, in short, the air filled tower sound like a dud idea to me as far as a means of access to space, probably a dud from atmosphere research as well.
