I don't think there's a best pipe period. Every motor is subtley different. Buddy of mine with a 10mil cub lost 3HP and .1 in 300 feet with the GRR outframes and went back to his shearers. Then there are plenty of other folks that have good success with them.
As far as the GRR inframe, i'm sure they are gonna be like CPI's or shearers and want a high exhaust duration. They'll probably perform similar to them, but personally, I don't like the mechanical design of them. The shearers are like T5's, where they curve down and give you access to your plugs and provide plenty of clearance for fuel lines and air filters. All things being equal, I'd take a set of shearers over them, and I've yet to see anyone with a dyno or other proof that even shows them being equal to a set of shearers.
I ain't hatin' on GRR. Have never dealt with them, just giving my .02