I think it's the rare person who can implement 100% of GTD or any other time management system.
If that's true, then it's probably better to ask "how can I make sure my system is meeting my needs?"
It's a tougher question to ask, but the answer can take someone from GTD to any other system that exists in search of possible add-ons, ideas, modifications, etc.
The reason that this might be better approach is that before encountering GTD we each had a system that worked to some degree (as defined by our habits at the time.) We found GTD, and made some improvements... but in the end, the system still belonged to us.
While it's tougher to think this way, I believe it produces better results, as it keeps the goals we have in mind in front of us, and reminds us that we are always working on our own habits, practices and rituals -- and don't need to pay too much attention to how closely we follow any particular system, as long as we are getting the results we want.
Hope this helps,