I would argue there is a huge difference in pressure performing for Napoli than for PSG. Napoli, even now, are not expected to beat the likes of Juve. If you're leading a line for a team that is fancied, not favourites, you won't have to play under the weight of expectation that comes with knowing any loss is unacceptable. Having that freedom seems to allow some players to express themselves more, Cavani isn't the only one. Many a player has looked unstoppable at a club that is expected to compete but not to win, only to move elsewhere and freeze under the spotlight. Just look at how Torres retreated into his shell after moving, from a Liverpool side nobody really expected to win the title, to a side where even winning often sees managers get fired. In fact it even happens to managers e.g. Moyes.
Cavani may get moved to accommodate others its true but he's also had some real stinkers, games especially for the national team. The last Confederations Cup was a case in point. He wasn't marooned out on the wing. He had some really good chances and missed them too. Cavani operates well when expectation isn't on him. He visibly finds it hard to cope with being in a situation, not where he's the main man, but where he is expected to deliver victories as a matter of routine e.g. at the very biggest clubs. I don't think he's done enough to PSG to moan about the formation change, he wasn't playing so well that it was unthinkable to ditch the 4-4-2.