Lance, you might want to try some different fuel.
You and I run very very close to the same set-up, 'cept for your bigger turbo. I've always prefered to run timing at 29/27 for many years, and I DO LOCK my torque converter. (See our mph difference)
Of course, there's always that pesky altitude thing too.
I've tried on a couple of occasions running VP-C16, but had almost the same problem you do. Had a nasty but persistent knock retard showing up in 3rd gear. Mine on most times was a little different though. Had a spike at the shift into third, and another biggy at the TCC lock up point a few seconds later.
To my good luck, I couldn't get C16 one day and went to another local supplier and got some Unocal 114. Since that day, I've never looked back. I can't pretend to explain why the Unocal 114 is so much better than the C16, BUT IT IS.
Now with my timing in the 29/27 setting (my extender is a 23/21 chip, so I'm running the base swith in setting 3), I never see any knock retard at 25# of boost, and only when I start pushing 27# of boost do I start tickling the knock sensor.
May or may not work for you, but I'd recommend trying something other than C16. I was told by a "race fuels guy" (paraphrasing here) that the reason you start loosing performance/anti knock ability, it the very high octane fuels (approaching 118) are for very very high rpm engines (like pro stock) and as such, burn very fast. Too fast for our LOW rpm applications.
Made some sense to me, and may be why my car has never like C16 as well as the Unocal 114.