There are several things here:
*You want your air to move as easily as possible = use the biggest valve you can, and the biggest pipe/tubbing from the chamber to the valve to the barrel. In a perfect world, if you were using a 2" barrel, a 2" valve and 2" pipe from the chamber to the valve would be great. That may not be possible though.
So, your 10 feet of 1" tubing is costing you a lot of power.
*You want to release all of your air at once+ get the fasting acting valve you can find. A good solution is a sprinkler valve. These are electrically controlled, so you could use them with the FRC robot components.
In a perfect world, you might use a large chamber, have a pipe come out of one end the same size as your valve, have that pipe turn 180 degrees, go into the valve which then goes right into the barrel.
Really, you want as little extra pipe as possible.
These t-shirt launchers are a lot like pneumatic potato guns. I suggest you explore this site for some quality information:
This is a god example of an efficient gun that you might consider.
http://spudtech.com/detail.asp?id=52
You may want to peruse the rest of the site for more information, especially the "links" tab.
If you want more information about optimal chamber to barrel size ratios, PM me, there is much more info out there.