I realize this is an old thread, but just in case it helps, the best solution would be to overlay multiple objects, each with a relatively mild background blur effect applied.
I think I missed this post back in June. I just saw it yesterday and tried the advice. Works a treat!! I used 3 objects with fX & fY values of 0.3, 0.5, and 0.7, respectively. With this combination, I achieved basically exactly the sort of super soft blur I was after. Having 3 fullscreen shaders active takes a toll on performance, but for a pause screen that's not an issue. I'm really happy about this - thanks MuddyMole!