I haven't yet tried 3.1.5. This unit has never been hacked and I have a new drive on order that I plan to do expansion and hacks on. I'm waiting to pull my drive until the new one arrives. I'm surprised to hear the 3.1.5 kernel didn't work for you, it was my understanding that its been working in all 7.x versions so far.
I will get back when I have some results to report.