I was never able to get priority coding of any sort to work, even before I discovered GTD. It wasn't until I read the book that I realized why: because my priorities shift from hour-to-hour or even moment-to-moment in some cases.
I've read a suggestion from Kelly Forrister that was a huge help: when trying to decide what to do, ask yourself two questions: "What's the payoff if I do this? What's the risk if I don't?" That works for me because the answer to those questions can change at any moment depending on what is (or isn't) happening in my work and life.
I find the different models for choosing what to do work perfectly for me. I particularly like the four-fold model: I narrow down my actions first by context, then by time available, then by my energy level, and then finally by priority. I usually pick four or five at a time and when they're done -- assuming something else doesn't come up that should take precedence over my pre-defined tasks -- go back to the ocean of my next actions and scoop up a few more in my bucket.
If you can make pre-defined priority coding work for you, more power to you. I've never been able to make it work. But I was able to make the GTD system work.
So glad those questions were useful for you! I have personally gotten so much value out of them too.