Craig,
I have a 10:1 compression engine and I'm not 100% comfortable with the detonation margin.  But there are different combustion chamber shapes in the various engine, so results will vary depending on the engine.  The octane requirement goes up out of proportion to compression ratio, but the incremental benefit keeps getting smaller.  I think if I were doing it again I would settle for 9.5 as being a reasonable compromise.  I understand Lycoming builds helicopter engines at 9.5, but they run at higher rpm so are less susceptible.  And a Lycoming rep told me that the crank and main bearings are not likely to last 2,000 hours at 10:1 (a statement from their legal department?).  Some have said that in some engines the timing should be set at 25 degrees with 10:1, but I don't believe that.  I had one that was set that way and I could hear an occasional audible knock.  I now have the Lightspeed set at 22 (can't retard it any more than that) and the mag at 20 and so far, so good.
Gary