I've been doing GTD for over 4 years now and the most surprising thing after all of that time is not how much more I'm getting done (though I *am* getting more done) -- but how much more I'm collecting and processing. I often process 70-100 new thoughts a day out of my inbox; and handle an average of 70 projects and 250 actionables a week. The problem is you can always collect and process more than you can do. And at this stage in my journey I'm realizing that there is a certain place I must come to in accepting/resigning myself to that reality. In this case I think that while the system might be able to be improved upon, I might learn something new I didn't realize before, etc. the basic reality of always being able to collect and process more than I can do confronts me with a response of either denial or acceptance -- accept my own limitations (I can only do so much every day) and resign myself to the fact that this is okay.
It's a stage in the journey I think. We begin with an almost idealistic optimism that GTD will be that magic fix for us. And this optimism may be what we need to propel us into learning the habits. I remember thinking that GTD was going to solve all of my productivity woes. But eventually one moves beyond this stage -- and the youthful confidence is replaced with an aged wisdom; a wisdom that looks over the vertical and horizontal landscapes with the knowledge that, at most, it can only rule a city and not the world.
Hope that helps.