Different exhausts make different power levels due to pipe design and diameter. Slip-ons make the same amount of power as stock because the factory headers are optimized for emissions not performance, and therefore choke the motor. Any gains that would be realized by the larger diameter and freer flowing nature of the slip-on pipe are negated by the factory header design by the time the can is reached. There is not nearly as much baffling in an after market slip-on as a stock muffler, thus the increased noise, but the performance damage is already done.
A 4-2-2 design can make as much power as a 4-2-1, but will be down on top end hp from an equivalent true 4-1. Conversely a 4-2-1 or 4-2-2 will have a slightly higher amount of torque due to the increased backpressure inherent to the design.