My wife and I own several restaurants as investments (we don't actually work at them, and neither of us is a chef, but I spend a lot of time hanging out at them because I enjoy food and because I like to keep an eye on our investments). Certainly some things are par-cooked in most restaurants (chicken wings, for example, are best fried at a low temp for a while and then finished later at a higher temp to crisp the skin), but I have NEVER heard of par-cooking a steak. I'm not doubting that it happens, but I cannot imagine how it would taste good or be a good investment, since it seems like it would lead to a lot of waste if too much is par-cooked and needs to be thrown away (assuming they are following health codes for safe holding times, that is).
As another poster said, I think it's more likely that they constantly have certain items on the grill so the food comes out quickly. Unfortunately, while this is good for table turns, it's terrible for the guest experience. If nothing else, the server should know to slow things down (although obviously a server focused only on table turns may not care, which is at the root of the problem).