Interesting information there about not being able to feel a difference in power. I did a bunch of searching online about tuning HD gas trucks and most 6.0 LS guys run about 26-28 degrees and 8.1 guys run 20-24, but they also zero out the IAT and burst knock modifiers that bring the actual logged spark down to about 10. (Also enabling PE).

I know with my Vortec-headed SBC in my old truck, 28 degrees made good power, but that's as far as I wanted to push it. I had 30 degrees in my 10.5:1 Vortec-headed SBC Camaro and it was great. From what I've read that's 6-8 degrees less than older open chamber SBC heads.
The guy who sold me the heads and cam for my 3.4 V6 said 26-28 is where they make the best power. They are all a long way from 10-15 degrees...

Here's the truck!