I like my 916's they are considered a mid range pipe great power and I haven't ridden with a faster accelerating quad but I admit I've only ridden with Raptors, Blasters, 400EX's and Polaris's so I can't compare to another shee. But it's like a rocket compared to stock.
I guess the theory behind the 2 into 1 is similar to adding an H or X pipe to a 5.0 Mustang. Torque gets you there and Horsepower keeps you there.