Exhaust system implementing feedforward and feedback control

X X An exhaust system for use with a combustion engine is disclosed. The exhaust system may have an exhaust passageway, and a reduction catalyst disposed within the exhaust passageway. The exhaust system may also have a first sensor located to generate a first signal indicative of an operational parameter of the reduction catalyst, and a second sensor located to generate a second signal indicative of a performance parameter of the reduction catalyst. The exhaust system may further have an injection device located to inject reductant upstream of the reduction catalyst, and a controller in communication with the combustion engine, the first sensor, the second sensor, and the injection device. The controller may be configured to determine a NOproduction of the combustion engine, determine an amount of reductant that should be injected based on the NOproduction and the first signal, and adjust the amount based on the second signal.