RE: Brushless or Lipo - one or the other
Leave the 350s installed. The 400s are slightly heavier and are very amp hungery from what I have been told. I would just go to a 2s Li-Po and leave the brushed power system in it for now. If you go brushless then you may want to step up to Li-Po much quicker.
I knwo alot of people are flying them with the brushed motor and the much lighter Li-Po packs. All these people have reported much better flight with longer flight times.