This paper deals with the quantification of performance bounds in multiple-input multiple-output (MIMO) control systems when energy-constrained input channels are added to the plant. We focus on stable, discrete-time, linear time-invariant (LTI), and right-invertible MIMO systems. The performance is quantified using the 2-norm and assuming step references, whilst the control input energy constraint is on the increments of the additional input signals. Our main result is a closed form expression that quantifies the improvement in performance that results as a consequence of the additional input channels. (C) 2014 Published by Elsevier Ltd.