My opinon is that is doesn't make a difference once up to speed. With the engines and clutches we run it may take more for your driver to get up to speed but a top speed will still be achieved at either weight. One advantage is like stated if the track doesn't have alot of grip your kart will handle better, vs when it's hot sticky the lighter driver has the advantage. I know to some people a tenth of a second is huge deal. To me it's not, I've unloaded with drivers and been way off the pace to start the day. By just working the chassis setup we've picked up over 4 tenths or more on some days and took home the top prize. So it all depends on your attitude and work ethic to me. My daughter currently runs 11 pounds heavier with no added weight in her class. There's nothing I can do to control that, it's the way it is. The first race of the year she was a tenth and a half faster then the kid that was right at the minimum because of setup. This is also in a class where the engine is purely stock and rpm limited equally so the kids have an equal chance at running up front and winning the race. It's in the setup of your chassis and how you transfer the weight IMO.