Regarding W/P8 sounding bright... To my ears they sound little bit "hot" in lower treble(not high treble). I even personally discussed that "quality" with Peter Mcgrath on one of his presentation of W/P8s(he setup the system with W/P8s on that presentation)... So, we can asume that prominent lower treble is Wilson sonic signature.
I wouldn't say you have a good rationale for making the above assumption. My measurements and my room don't reflect those qualities you describe.
My guess is, it was just a bad room without adequate lower treble room treatment. I've noticed that Wilson only puts up a curtain in its demonstration rooms, which would not be adequate room treatment for anything but the highest frequencies. You aren't going to hear how great a speaker sounds unless you put it in a well treated environment. Unless the speaker is rolled-off at the higher frequencies, you will get a cacophony of high frequency sound in untreated or partially treated spaces. The exact frequency spectra being affected will be determined by the frequency ranges not being acoustically treated...
I've heard demonstrations at shows, et al., as well and I know what you are referring to, but that is an issue with the room, not the speaker. I've also been to dealer demo rooms that were properly treated and the difference is stark. Not to mention listening to them in my own room...