It seems Superior Whirlwind Attack is buggy. Last night Kaos got the feat (character is an epic/mythic WM, took it at level 42, has Improved Whirlwind Attack), but it doesn't seem to be showing up anywhere on the radial menu at all. He tried un-quickslotting it and relogging, to no avail. When using the Improved Whirlwind Attack (since he could select it), it did none of the additional damage/etc. that's supposed to happen with Superior. It is appearing in his feat list, so it's not the level itself acting funky.
My Edon Characters"Tomorrow will take us away, far from home,
No-one will ever know our names..." --Blind Guardian