Mendel, I'm 6'2" 200# and I ride a custom 162. I'm listed as just outside the weight range for that board, but I love the way it rides. The weight range is just the company's way of saying "If you want the board to have the flex we say it has, you'll need to be in this weight range, if you're outside of that range, it may be alittle different than how we describe the board in our catalog"
At 200# I could ride a custom 152, but i'd just have to understand that it will be super soft and flexy under my weight. Conversely, a 120# kid riding my 162 would still be able to ride it, they would just have to understand it would be super stiff. Neither of which is how Burton describes the Custom.
I notice the speed picks up quickly once I do that; so I should keep my center gravity on the middle of the board?
Exactly. Ideally, your weight should be centered over both feet. It's a super dynamic thing, and it may change slightly throughout the turn, but I'll tell you the reason I say ideally is centered, since I faced the same problem at first when I learned to board.
When we are newbies, we tend to have speed anxiety, we're simply not comfortable picking up much speed. Because of this, any run with a decent slope causes the same anxiety. Don't think of yourself as a puss, it's just your brain's self defense mech. So we lean back......a lot. And typically ride with a very stiff stance. Since turns are initiated with the front edge of the board, it makes it pretty hard to get that edge to initiate the turn when you don't have any weight on that part of the board right? A nice athletic stance helps two fold, it makes turn initiation easier, and also it creates room for error, you can absorb bumps and potential caught edges, because your joints are bent, and they can absorb and correct whatever goes wrong. Hopefully that all makes sense?
Snowolf is dead on with regard to the affect stance width has on turning. Once you are really comfortable on the board, and don't have to think about the mechanics of your turns, you might want to start fine tuning your stance. I kept widening my stance until I point I noticed it was slightly harder to initiate turns, and then went back to setting right before that. For me it's 23-23.5" but I have long legs (6'2")
Moral of all this, just get out there, ride, and enjoy your new board