Part of the thrill of snowboarding is the danger! Yes, you may feel more comfortable going faster, but at the cost of reduced peripheral vision, reduced noise, and also you won't have that great feeling of the wind hitting your face! I've never even seen a full face helmet on a snowboarder (apart from someone trying to set a world speed record!).
Now, I'm not saying don't wear a helmet, I wear one for 90% of my riding nowadays and I feel that I can push myself harder, but for god's sake don't overkill it! A full face helmet is only going to protect your jaw, and if you break your jaw get some good photos of it and show it off on facebook, that story would rock and you'd look like a bad ass!
The whole reduced vision argument is complete bullshit.. same thing motorcyclists try to say. If I can ride at 130+ mph at Road America with a grid full of other bikes and have enough vision to do so safely, you can certainly do it at 40mph on a snowboard. Now with saying that, I don't wear a fullface but ride with a chick who does. If it makes you feel safer and you're having fun go for it.