SpudFiles

Yup we're very familiar with flexible diaphragm designs.

Sounds like you've got a plan going now.

After you've made your sprinkler valve cannon I would suggest taking a look at piston valves if you're really after speed and power.

If you're not into building them then a piston QEV is a easily installable alternative (see signature cannon) with three threaded ports.

If you're purely interested in performance, I would agree with the triggered burst disk suggestion. It's relatively easy to build if you can source the right parts, might be a slightly greater pain to reload than a conventional valve but for sheer muzzle velocity nothing comes close.

You can find all the details you need to build one here in the wiki