For your strategy (science vessel plague) There is a much more effective way of rocking the drones without using too much irradiates... all you need are 3-4 vessels and use defense matrix on as many as you will bring to your opponents base. Then use irradate on your science vessel(s) then go in to your opponents peon line, then fly your vessels right over them. So a huge portion or their workers would be destroyed without having to spend an extra few hundred gas and mins to build more vessels.

This ony works against terran scv's and zerg drones... and also any other flesh type units... eg) marines, batz, etc...