I've been trying this out on the whole screen on a 1024*768 example with alpha channel AOs.
As you can imagine, the performance drops like a stone. <img src="/center/images/graemlins/smile.gif" alt="" />
However, that needn't be a issue if you just want a quick burst of sunshine. The problem is switching it off again. It seems to need a real event to change the values - decrementing a counter does nothing.