I've been looking at this sort of thing recently. I have a mechanical support/resistance system for cable and I was curious to see if it would improve if I kept out of the market around announcements.
So far I have just programmed in (in Tradestation) the 1:30 pm announcement dates so that if there is an announcement that day I won't enter a position between 1:00 and 2:00.
I haven't so far filtered the announcements as to whether they are (considered) important or not - I just included all of them.
The result is (obviously) a reduction in the total # of trades but a modest increase in the average trade size and PF. My conclusion is that the improvement is signifiant (i.e worth doing) if not dramatic.

This is for a short term supp/res scalp strategy though. I haven't tried it on a breakout system that might have a longer holding period.