I couldn't agree more. The top 6, including Roddick, for sure, are the real deal in my opinion. Del Potro and Djokovic are the up-and-comers in that league, though, and I think they could have an extra good year in 2010...they'll make the others work even harder at the slams. It'll be an interesting year!
You did an excellent job with this comparison. Nice to read a well-researched post!
I also like ledigs' thoughts that when these guys play each other they get "practice" in a way they don't get with others. For a recent example of this, consider Nadal's match with Tsonga versus his match with Djokovic (though I prefer to not think about it - oy!)...Tsonga's good, no question, but not as good as Number 2 and Number 3.