Modern clearcoat is essentially plastic. I've had games cleared by just about everyone and the ball is insane until the surface dulls a bit.
I have played Playfield protectors/hard tops on lots of classic games. Yeah they play fast until they dull up a bit too. I don't really see a lot of difference.
Now if I had 2 games in front of me for the same price, same visual condition, one that had a trashed field replaced with a hard top and one that has a tiny bit of insert wear... All things the same, I'm taking the insert wear.
Questionable clear or clear over bad touch up? I'm going hard top.
I would rate hard top as half the distance between trashed and mint/fully restored. This gap is further apart on some games than others so I don't think it can be looked at as a flat value.
Sorry for adding to this nonsense