Yes indeed I should check that PSM is improving the situation. I actually put my phone on PSM after 5mn iddle (thanks to your app) and even not sure now, I do not think it helps enough.
If wifi off is not possible (that was possible for the N900 i think but a less restrictive phone for such features), do you think that the fact that a 'known' wifi network can be either automatically connected or manually connected is helping and could be implemented in your app?