As diesel engine emissions standards become increasingly stringent, a commonly employed method of emissions reduction by engine manufacturers is active control of inducted air and the use of EGR. A common system configuration includes actuators such as an EGR valve and a VGT are used to manipulate the air flow through a diesel engine to control desired in-cylinder conditions so that the combustion event produces reduced engine out emissions. This paper evaluates four different controller designs for control of a diesel engine air path using a VGT & EGR valve in order to explore trade-offs between system performance and system complexity: three built up from SISO transfer functions and one that is a fully multivariable design. Various performance metrics are analyzed to gauge the relative difference in performance capability while attempting to maintain simple controller architecture. As part of the analysis, the controller designs are simulated with a realistic non-linear engine model against an excerpt of a standardized certification test cycle to give a sense of how the controllers would perform in a real-world application. Our evaluation indicates that a capable and simple decoupling architecture can be designed, showing some performance advantages over a baseline diagonal architecture.