Ive been messing with sliders as well trying to get guards more rebounds and bigs less rebounds.
Ive lowered rebounding down for boht off and def, some and that hasnt done much really.
1 thing I would recommend trying is lowering hands slider. Big typically have lower ratings in this area, which they should, and this seems to help, or I think it has anyway. Oddly enough you need to raise the slider that protecst the ball, not ballhandling, but the other 1 thats right above the collision slider, otherwise you will see too many steals. I keep ballhandling low, as you see more charge calls this way.
Something else I have noticed is some big have higher rebounding ratings than they should. For example Roy Hibbert and Rick Smits on Pacers all time team. Neither were good rebounders when you take into account Roy is 7'2 and Smits is 7'4. doing some editing to fix rosters does make a difference. Not sure why roster ratings are always so bad each yr with NBA2k, I sometime wonder if those who decide these ratings, watch much of the NBA.