It's not just thread depth that you have to consider, but how deep the groove is for your locking washer and the chance that you might deform the thinner steer tube walls with the expanding base of the quill stem.

I remember a while back (more than a year) someone on here talking about having a frame builder modify their Soma fork by cutting it close to the race and welding on a thicker steer tube that had the correct tolerances for such a task. Might be worth a search.