Physical Strength *can* be maxed, yes, but its damage output is somewhat lacking and it is just plain annoying to combo with. It does allow you to actually let the spacebar-followup connect in mid-air though.
Whirlwind seems to me like its subject to preference. Maxed Whirlwind does do nice damage, but in order to get all the hits in, you'd have to be in contact with the enemy for a pretty long time. Usually unflinchable mobs don't give you that amount of time. Of course, it's great against comboable enemies, but they get increasingly rare as you progress through the game.
Also, the reason to max Boomerang, is that it gets a really noticable damage boost at lv 68, doubling its damage.
According to the latest skill calculator, Disturbance gains a second of side-effect per level, so 12 secs when maxed.