I A/B'd both as well (along with the TH900 and E-MU Walnut and Denon D1000) recently.
I 100% agree with your take on the treble. Absolutely spot on there. It's a lot of treble, but it is perfectly smooth. Whether a listener thinks it is sibilant will come mostly down to how much treble they can tolerate. I think it's technically not sibilant, but most people say sibilant when they really mean hot treble. It isn't harsh and bright, which to me is what I think of sibilant. It's smooth and bright int he treble, airy at times.
I disagree slightly in the mids, but I think that may be something of what we are calling mids and bass, rather than a difference in how we actually perceived their sound. To me the Teak had sucked out lower mids, but the upper mids were forward like you described. To me the Teak had more lower bass, but less mid bass than the Fostex. The Teak, to me, was more focused around the edges of the frequency response, while the THX00 had a thickness in the lower middle.
A lot of the difficulty in comparing them comes from precisely volume matching though. It took me a while to get their volumes matched, due to the Teak being slightly more efficient than the THX00. At first I preferred the Teak, but then I realized that was just because they were louder, when volume levels were matched, my preferences tipped a tiny amount the other way. I slightly preferred the signature of the THX00, but I could certainly see where somebody would prefer the Teak. I'd maybe say the Teak is a slightly better technical headphone, with better extension, soundstage and resolution. But the THX00 is very close, and preferences will probably come down to subjectively what the listener wants signature wise rather than the relatively small technical advantages the Teak has.
My take was that the Teak essentially split the difference between the signature of the THX00 and TH900 signatures.
All amping was done with my Grace m9XX or directly out of an iPhone (I have had my THX00 reterminated to be 3.5mm, so no usage of the crappy adapter, the Teak is already terminated in 3.5mm).
By the way, I have had it confirmed to me that the driver in the Teak is the same driver as in the TH900. The differences in sound between the two are mostly due to differences in pads and a small difference in the cup shape (though not the type of wood, which I was told has no measurable effect between units). There is a difference in stated efficiency, but that is apparently due to some tuning done on the cup shapes of the Teak. Also, some people have said that the TH900 is really more like 103dB anyway.