Tested on 64x version of CoH2:
Garrisoned Royal Engineers in mortar pits (done testing in two different MPs both with a garrison and without), and tested the rate of fire with a stopwatch. Each time, it took around 10 seconds between salvoes, regardless of whether a unit is stationed within, though the icon of the buff was lit up.
Please tell if any additional info is needed.
(also, when clicking on one of the 'popular tags' when creating a forum thread - 4 of them are added at once instead of just one)