Yes, casting time is important - and sometimes a life-or-death situation - but I still believe that having twice the number of PP is more than getting a few +5's, even if you get them so they stack. JMO. (I think that the biggest benefit to having some +5 items is that they are worth a lot of $$, and with the right character can net the group a serious amount of cash they can use to stock up with.)
That idea for an item that adds to the character's level for casting purposes is nice. STEAL! (Alright folks, nothing to see here. Move along, folks, move along.) Of course, I will use it for more than just spell preparation purposes...I will use it for all aspects of spell-casting: duration, range, etc.. This sounds like what a Staff of the Arch-Magus should be able to do, among other things.