I actually am having the same issue with my IS. I have the SPEC stage 2+ clutch with the Fidanza lightweight fly. When I pull off from 1st gear and rarely when I shift to 2nd I get the same very loud squeeling noise.
I still haven't determined where it is coming from, but it doesn't affect my performance or drivablility. Initially, I thought it was the fact that I have an unsprung clutch and unsprung flywheel. But... when I took my car to Dyno it, the guy owner of the shop (and avid racer for years... supposedly) said it was "Nature of the Beast with Carbon/ Kevlar cluthc discs". Skeptical, I called SPEC and they confirmed that the clutch discs are made from Carbon/Kevlar.
The owner of the shop also informed me that with that setup, the discs will wear slightly quicker than normal clutch discs. This due to the speed of the flywheel (being lightened) and the carbon/kevlar slipping momentarily as it makes contact. A gentleman getting his car dynoed after me said he had the same thing on his Porsche (except it was strictly used for racing).
With all that being said, I'd suggest you check to see what your disc is made of and let me know. If you too have a carbon/Kevlar clutch disc, I would pretty much accept that as the root cause. Only real way to find out, would be to drop the tranny and see the wear pattern and replace the clutch, which I doubt you want to do.
hope that helped! Let me know what you find out.
