38-deg advance sounds about right for light load conditions and medium rpms. I'm sure you know (as I) that TPS position doesn't necessarily correspond to load conditions. Going up a hill with 25% throttle requires difference mapping than 25% throttle downhill.
Assuming the physical load on the motor didn't change drasticcally, I agree that a drop from 38 to 16 degrees (with only 3% more TPS increase) is perplexing. The only place my SBC has advance that low is in the lowest rpms for maximum load (to avoid knocking during anything resembling engine lugging.)
In a quick search thru this forum, I didn't find anyone knowing of a successful ECU reprogram. Do you know any way to hack into the fueling and/or timing tables? What tool(s) do you use to monitor the conditions you're seeing.
BTW...Interesting that you say "My bad" in Oz.