To move a pneumatic cylinder quickly, you need to get rid of as many things that slow down the flow of air as possible. Pneumatic tubing, the primary regulator, and the solenoids all restrict air flow to some degree. If you can optimize each of those to flow more air, then you can make you're catapult cylinders fill faster and deliver more energy to the ball.
I've posted about this before (see
this post), but the general idea is to minimize the obstructions that the air has to go through on its way from the storage tank to the cylinder. Use short lengths of tubing, get high flow solenoids, and put an air tank right before the solenoid so that it doesn't have to go through the main regulator.
The solenoid you're using has a Cv (basically, how much volume can flow through it in a minute) of 0.22. McMaster has a solenoid that we like with a posted flow constant of 0.75. The solenoid might not be what's restricting you, though. It might be the tubing and the regulator that's taking away most of your flow.
We did a few tests with prototypes using our maxed out flow setup and found that we were just reaching the high goal from the outerworks with two 1.0625" bore 6" stroke cylinders. Smaller volume cylinders weren't quite making it. I suspect that you might need more volume in your cylinders than you're currently using to put enough energy into the ball.