Glad to hear your 9.5 works well for you on your 138. I just went down from 158 to 148, so as I mention above, it may just be that I haven't found the sweet spot yet for tuning mast track position, boom height and harness lines. It sails okay, just doesn't feel as balanced as the 8.5.
I also have an HPL boom head (the Streamlined knockoff one) that constantly slips, on tacks and pumping, so that doesn't help. My dealer was working with HPL to fix the problem, but after months of waiting, I gave up on HPL and ordered a Streamlined head. That will be in this week. I would bet my tuning gets easier once I have a stationary boom head.