Thanks for the advice. The trouble is, the owners are not comfortable with me doing anything complex to their engine myself (I've done compression tests on my older legacy). I could arrange it with some shop, but how do I trust them to do a good job? I don't know any good ones (I've always done all of my work myself). Plus, a compression test is about engine wear, not as much turbo wear, which appears to be the concern here. They did not want me to open the exhaust without some guarantee and I can't blame them.
Here's a thought. If it's all about losing the turbo (as along as it's caught in time), then, given that a new turbo is around $1200, if I can lower the price by that much, maybe it would make sense. As long as I can figure out a way to get a warning (that my wife, who'd also be driving the car) to react immediately upon a failure. Or maybe I preemptively replace the turbo?
This just stinks! I don't like the NA 2.5L engine or the H6. And the one thing about my Subies (all 650,000 miles of them) is that they have never, never, ever left me on the road, with all the abuse I have inflicted upon them (autocross, rallycross, ice racing, etc.) This is driving me mad!