Rob, you post a great question with this topic, one that has me really thinking. I haven't been on these boards for about 2 months now because the posts were becoming way too social and not very informative. But now you pose this great question....
I admit I'm one of those "best notes" people, in that I have my own criteria or "subset" as you put it, in choosing notes. The challenge then, for me, is to beat the averages which I have consistently done for 2 1/2 years now. This is a hobby for me, so I've mostly enjoyed the note selection process and I still do most of it manually. I have been using filters at LC somewhat the last several months though as it has become increasing difficult to get those "best" notes if you're not fast enough. But I use no filters at Prosper, and still hand pick (after using a general filter) all my notes at LC.
So personally I do think there is a subset within note grades (that's just my opinion) that can outperform the average in that grade. I have nowhere near the skill set of many here who can run numbers, accurately configure probabilities, or create API's or other software, but I still do okay.
I have 3 P2P accounts, and even with the many defaults I get because of my high risk exposure, my current combined weighted average return for those accounts is currently 14.72%. I've hovered around 15% for the last year and am currently at about the lowest return rate I've been at in 2 1/2 years. The first notes I purchased will start paying off in the next 6 months (for those that go full term), and my 3 year experiment with P2P should yield some interesting results. I am impressed with it so far.
Maybe I've been lucky, maybe I've stumbled on some skillset I don't know I have yet, or who knows what, but I will continue doing what I'm doing as long as I continue to get these results, which means that I believe there is a subset of notes that will outperform the averages.
All that said, I don't like all the defaults I get, I hate when borrowers make less than 10 payments, and in some cases they make only 1 or 2 payments and then pay off the note - those things waste my time and frustrate me, but I've learned that it's just part of the game and you have to accept it.
Again, great question........