This is a different way to look at the problem and I think it eliminates some variables if you like WinShares.
Look at each draft slot and get an expected win share for each, so the #1 pick should get you say a 7 win share guy, the #5 gets you a 4 win share guy, at 20 you are lucky to get a 1 win share guy, etc Check to see if the expected win shares from draft picks is actually reflected in the teams future record. So in isolation you would expect the #1 pick to blossom in year 3 and increase the win total for the team by 7 games from the time they were drafted. The formula is probably more complex than that, and it doesnt hold free agency moves constant, but its a framework to measure if getting better picks is indeed helping teams win.
You can also use the same calculation to look at GMs and say 'Danny Ainge is a +1 win share drafter', 'Michael Jordan is a -3 win share drafter'. If you did this for the GMs I think it would be very telling. If GMs stayed around long enough we could look at things like 'How effective is 3 consecutive lottery picks for a +1 win share drafter compared to a -1 win share drafter' and its this piece of analysis that gets to the crux of the issue for me. A competent drafter can bottom out and create a new high quality team. Michael Jordan could get a free lottery pick from in perpetuity and I dont think he could ever build a contender.