It could definite be a problem for casual players.
Sure, you will be overleveled later in the game. However, you are expected to be underleveled when you just arrive on reaper's coast. I have to explore and do all quests that do not require combat to catch up before I am able to finish higher level contents. It would be unwise to nerf exp gain at this point.
Larian should have nerfed the exp gain from contents in Nameless isle and Arx instead. There are definity ways to cheese arount this like:
Lava patch at cloisterwood + terrain transmution
but people would have a harder time if they don't like it.
