What you really care about is probably power to detect functional activation. Here are the results of a random effects analysis (N=12) of a motor cortex localiser task. Subjects were asked to press a button either with a finger of their left hand (left column) or their right (right column). The data were analysed with or without undistortion.
Functional images (thresholded at 0.001 uncorrected) are shown superimposed on the SPM T1 template. The numbers in the top left of each image indicate peak t values over the whole volume. After the application of undistortion, clear activation can be seen in motor cortex (BA 4) on each side.