The regular game is the same difficulty, it's just the new optional bosses and the bonus dungeon that are more difficult. I had no problem with the main story bosses, and I hardly ever fight random encounters unless I want to (on Proud Mode). Now, if I were playing on Critical Mode, that would be a different story.
Roxas was disappointingly easy, but the battle was still really awesome.
Here's a battle with Larxene, and
this is the new Roxas battle.