Logically, it should be comparable to the overall penalty we originally got out of fatigue, but the other way around.

Basically, someone playing 10-15 hours a week would never hit fatigue, as the same person could very well be in permanent rested bonus if he/she rests at every log off.

Up to a point tho, exp gains would be cut by up to 50%, which correspond to currently playing without bonus. What was this exp threshold again? I'm not saying that the numbers should be the same, especially that the bonus is applicable to all classes whereas fatigue was class dependent, but my guess is that the balance should be somehow comparable.