There was a thread on this last year
http://custombats.co.uk/cbforum/index.php?topic=41544.msg659921#msg659921
Not sure whether @swark did the research he was talking about, maybe he can update us (although doesn't look like he has been on since last August)
Still lurking about - I don't tend to log in, so it doesn't appear that I am around, but I am still reading.
The research basically hit a bit of a road block. I did some trials with various timbers, and the preliminary results were fairly clear that players could distinguish between some types of willow, but there was limited differentiation between the grades of English willow (i.e. grade 1 versus grade 4 was hard to tell apart purely on performance).
HOWEVER, there were some major methodological flaws that I couldn't really overcome easily. In particular, I found it nigh on impossible to get bats of the same dimensions. Therefore, there was a fundamental difference between the bats right from the start, and just using one bat of each type naturally means that a 'dud' could greatly skew the results. I also didn't come up with a great way of covering up the blades (to try and hide which bats were which) that didn't also impact on how the bat felt at impact. However, I have just recently seen bat wraps (do a google search for "bat wrap") that may work better.
I really need a bat maker to come on board and make up 10 identical bats from the different types and grades of willow, but understandably, the major players weren't too keen and it is a huge cost / time impost for the smaller guys (and I can't really afford to buy 10 bats that would ultimately be cut in half in the later part of the tests).
So, long story short, the project is still floating about in the back of my mind, but I need to win Lotto first. Very preliminary gut feel - English willow is still the best, but people may be surprised by how good and close to it Australian willow is. English willow nurtured in India - the jury is still out. Kashmir, hmmmm ...