I am a firm believer of neuroplasticity. When studying exercise physiology at UGA, we used this theoretical principle to help patients with multiple sclerosis to improve their motor coordination. You have to do both mental and physical training though. My doctoral professor published a few studies on the topic in peer reviewed scientific journals, and I've seen patients relearn how to walk, and in some cases run.
I personally believe this works in regards to any type of activity that involves motor learning. I've also read studies that show increased EMG activity to specific muscle groups when the subjects were told to think about contracting that particular muscle (even though they remained still). Therefore I believe there may be some motor unit activation, even if it's minor.
I do a lot of mental imagery/visualization training the week of competition. It's almost like meditating. Whether it's a placebo effect or not, it works for me, so I choose to believe it works. But mental visualization alone doesn't work, you def need to drill and train the movements in both controlled and live situations in order for your CNS and PNS to develop the neural pathways.
I.E. You learn a new technique, let's say an armbar just to keep it simple. You drill the steps several times one day and maybe even try it live (and if you're succesful when going live then it's even better!), then, after training when you are at home you visualize the steps over and over again. You will probably remember the details more when you practice it the next day and your motor units will fire more adequately. This may improve your technique on the following training session.
Of course this is all theoretical, but Couture does say that fighting is 90% mental, 10% physical. I believe there is a lot of validity to that statement.