to me an audible rush of air that doesnt result in any measured boost loss but kills power deffinately screams BOV..
I may be wrong - but wouldn't a simple way to see wether or not it's a broken BOV be to simply to disconnect it at one end or the other?
if there's no change in symptoms that would indicate that the BOV has gone bad.. otherwise you'll have to look elsewhere for a pipe that has blown loose
(any comments on whether or not that's a good idea are more than welcome - i havent had a problem with the vavle yet myself so i haven't tested that.. but it seems like that would be an easy method of diagnosis
)