Shapeshifters aren't terribly deadly in PK, at least not most of the time and until the higher levels. In that sense, it's fine that their penalty is zero and therefore have a "sweet range". That said, upping the penalty might discourage people from rolling shifter-after-roleless-shifter until they get nice forms. With the addition of the re-roll edges though, maybe that's not as big of a deal as it was before.

Nonetheless, a 500 XP penalty might mean we would see fewer Izuhrs and Ccurtohns, which I'm sure we can all agree is a good thing.