Most i;ve ever lost for a fight was 17.25lbs needless to say. You DO tend to feel like **** after that but the fact is its a HUGE advantage when you have the extra weight the next day. I usually use pedialyte to get back to my regular weight following a weigh in
Cutting a lot of weight doesn't necessarily mean you'll be crawling around on the floor due to exhaustion. Many professional fighters do it and don't look too bad. Obviously you won't be 100%. Go to sherdog.com and check out the pre-fight weigh ins. Guys like Matt Hughes, Rich Franklin, Georges St Pierre, Tito Ortiz, and Chuck Liddell regularly cut anywhere from 15-20 lbs per fight and don't look too bad come weigh in time. Many boxers, BJJ guys, wrestlers etc do the same thing. However these guys have built up a tolerance to cutting weight. They have all been doing it for years. What you saw on TUF is what happens when you're not used to cutting weight and cut a lot at once (ie 20lbs).
When I was in high school they didn't want us cutting weight, because of potential health risks, and that was when the kids in Minnesota (I think) died from cutting weight. We used to have meets that would be on the weekend and the weigh ins would be Friday and Saturday nights. If we failed either, we were disqualified from the meets. Since you can't wrestle effectively while you're cutting weight you competed at your "walking weight". Kept us honest.