I'm a classically taught violinist and a self-taught drummer; "mentally practicing" drumming is much more effective than violin. I've made most (+50%) of my significant improvements in drumming first in just slapping my palms on my knees and tapping my foot. Of course, it might just be because you can actually practice drumming with your hands and feet. Violin is much more difficult to mentally practice since you don't have the same feedback about how well-placed your finger is, but it still helps. Also, one thing that I find very difficult to improve with from imagination alone is keeping tempo.
Regardless of the instrument, I tend to have the same habits and mental processes when imagining myself playing an instrument as when actually playing it. For instance, when playing difficult parts on the violin, I will subconsciously and immediately flare my nostrils - probably conditioned into me over the years. Imagining myself playing those same parts also makes me flare my nostrils.