Going +1 is usually a question of reliability that can vary by specific mag used. So not surprising it wasn't mentioned in the manual. You'll have to run it that way on the range- do 1 shot drills and top off for each one, that will give you lots of proving reps without spending lots of ammo. It will also break in mags faster as you are working it repeatedly to full compression.
+p - I'd call or email them to verify. Are you normally running +p or is it more of a general question?