If you create a character on a preferred world you get a 100% XP buff up to level 60. I doubt they'll give any universal XP buff just because of the level gap; that's what those MSQ skip microtransactions are there for.

As for scaling skills while keeping the full kit available, perhaps that could be based on the average output of the character's rotation. For a very simplified example, take a low-level rotation which consists of spamming a single attack with a potency of 140. At a later level a combo attack with a potency of 200 becomes available so the rotation consists of alternating between the two. That's an average potency of 170 per attack. Now scale this rotation so that the attack potencies become 115 and 165. The average potency is 140, same as the low-level rotation, but the button sequence is unchanged.