I can absolutely verify this on PC. I have a Cavern Megabattle setup I use a ton, with lots of extra Marines behind a Ghost blockade. This feature becomes really obvious with things like the Scorpion's machine gun (or even its main cannon, for that matter). Shooting at the Marines that way is surprisingly difficult, as your aim seems almost entirely random and inaccurate; and then, all of a sudden, when they turn on you, your aim dramatically improves, and even the cannon seems to noticeably "lock on" to them, making your shots way more deadly. In general, vehicle weapons are more obvious this way than handheld ones (maybe that's just something about the third-person perspective), but it still applies in both cases.

Vale,
Perseus