Well, dual shunts supposedly do that....however, I've never seen a difference. If there was, you would think Reedy would make all his top brushes with dual shunts........I don't even think they make a dual shunt brush. Brush shunts only carry around 10 amps of current, most of the current is transfered through the brush hoods. This is why its important to make sure your brush hoods are sized correctly and clean....
The difference between a laydown and a standup is that a standup is taller than it is wide, so it will have less overlap on the comm. These are typically used in mod motors. A laydown, is basically just a standup brushed turned sideways....they are wider than they are tall. They have increased overlap....typically used in stock motors to provide more power....though some mods like the Reedy KR use them, as do some spec 19t motors..