It's probably not going to hold perfectly well at the margins - players on huge contracts probably do get a bit of an overpay because they are rare commodities, and players on min contracts are probably more likely to exceed expectations. (edit: As an extreme example, at his prime LeBron was dropping 15-20 WS per year, which would suggest that he was worth 45-60% of the salary cap. I don't know that he got quite that much. Lillard is currently being paid like an 11-12 WS player, and over his career he has generally hovered in the 10-11 range, so that's pretty on point. On the other hand, the John Wall contract has been an absolute disaster, but we knew that without resorting to any sort of analysis.)
For the Kings you mentioned (just this year, not going over the whole of the contracts):
Sabonis - 15% of salary cap, should be worth 5 WS, already worth 7.8 WS and on his way towards ~12 or so (massive bargain)
Fox - 24.5% of salary cap, should be worth ~8 WS, already worth 4.6 WS, so on pace for maybe 7 (slight overpay)
Barnes - 14.8% of salary cap, should be worth 5 WS, already worth 4.4 WS, similar pace as Fox, about 7 (decent bargain)