I have 700x23s now. Stock wheelset though. It would be nice to upgrade them too, but I suspect that is another small potato in the mix. I just passed the 10,000 mile mark on this bike so there are going to be a number of other items needing attention before I can address the wheelset.
Regarding tire pressures, I seem to recall discussions about that in the past and max pressure was not the answer to the best speed question. I don't recall the specifics right now.