Uncommon maybe but its definately done. There were rumors that Thiago Alves was walking into the cage at over 200lbs and going into camp even heavier. GSP usually walks into the cage at around 185 but has said he is bulking up to deal with the bigger WW's and the rumors going into the Hardy fight were that he would walk into the cage at nearly 195lbs. If that is true then we are looking at 13% or more than Lesnar would have to cut. Lesnar's team has said he walked into his last two fights at 265 without any cutting in the traditional sense.
There seems to be some confusion about the difference between the weight that a fighter "walks around at" and the amount of weight that a fighter cuts. There are fighters who will come into a training camp weighing 15% over their fighting weight (in the case of Anthony Johnson, there are reports that he's at much as 25-30% over when he starts camp), but rarely do fighters cut more than 10% of their body weight on the day of the fight.
I see very little reason as to why he could not make 235.
If someone like Brock Lesnar wanted to cut to 235, he's welcome to give it a try, but I'd hazard the guess that his camp would say its simply not possible for him.
Even using your 10% number we are looking at 258.5 pounds. With a one pound weight allowance we are down to less than 7 pounds or a mere 2% of his body mass. Lesnar may have to shed a few pounds at worse but making 235 from 265 is very realistic compared to the weight cuts of other fighters.
The thing is while a few HW's like JDS or Cain could possibly be considered borderline in the current HW field many of the smaller ones such as Kongo are fighting in the HW division for unknown reasons. Kongo wouldn't even have to cut weight he would come in 7lbs under. If Kongo feels his size puts him at a disadvantage for him the LHW division is easily reachable using your 10% analogy its still easily do able.
Cain, who comes in about 250 around fight time nowadays at heavyweight, would have an easy time, as he'd have to cut well under 10% of his bodyweight. The same is true for guys like Dos Santos and even Cheick Kongo, who are borderline.
It also lets those larger lightheavyweights who couldn't contemplate stepping into the heavyweight division before, because they'd be giving up 50 pounds, (220-230 at fight time vs. 275-2980, which is not uncommon in the current heavyweight division) step into a division where the most they're going to be giving up is 25-30 pounds, an advantage that we've seen in middleweight and welterweight fights before.
Your looking to fill the new cruiserweight division but neglecting the fact that the HW division would be hollow at best and dead at worst. The big LHW's are larger men than many of the smaller HW's. The fact some fighters are caught in the middle is not a probelem we have not seen at other weight divisions in the past heck Elite XC created a 160lb division strictly because Nick Diaz at the time was stuck in the middle were 155 was to hard a cut and he wasn't filled out enough for 170. The fact that Nick put on some weight and has even had success above 170 in Strikeforce IMO just proves that Elite XC's 160lb division wasn't necessary. Your trying to fit a division to fighters when they should be working on fitting themselves into there divisions.
I understand your math but if I am saying a fighter cuts 15% of his mass to make 235 you need to ignore 235. 270*.15 (15%) = 40lbs. Your looking at a fighter gaining 15% not losing it and the math isn't the same.
Actually, just so we're clear on the numbers, 15% of 235 is a little over 35 pounds. If you maintain that fighters can cut 15% of their bodyweight then they could, theoretically, come down from 270.
According to Lesnar's camp he isn't coming in at 280 anymore and considering Carwin was under the 265 pound cap before bulking up to fight Lesnar I see little reason he couldn't shed some muscle and return to his old weight.(Carwin was fighting at about 259 before he bulked up).
By the way, that excludes both Brock Lesnar and Shane Carwin (not to mention plenty of other true heavyweights) who come into camp at 280.
I don't doubt, actually, that Lesnar or Carwin could find ways to lose the weight and make 235. I seriously doubt that they'd want to. The amount of cardio that it would take out of them to make that weight, the amount of strength that they would lose, would discourage that cut enough.
The fact is thought that the major advantage these big guys have is the strength advantage they enjoy and even with losing some strength due to the cut they would still enjoy that advantage. Guys like Lesnar would lose there biggest advantage by choosing to stay in the HW division making a move down only seem rationale.
Who do they recruit to fill the void? Even outside the UFC its not like the monster HW's are everywhere. It seems to me that the last couple years the UFC has been looking for huge HW's but outside of Lesnar, Carwin, Tod Duffee, Sean McCorkle and Tim Hague they haven't been really successful.
See, I doubt that.
There are a substantial number of true heavyweights who have no interest in cutting to 235, and who probably physically couldn't anyway without serious health risks. Lesnar and Carwin would certainly be left before, along with many of the other true-heavies. Now, the problem is that there isn't a whole lot of good talent in that division. That's a real obstacle, but it's not insoluble. The UFC needs to recruit better in that division anyway.
How many MW's are moving up? LW's? WW's? Why are you so confident this divsion would be different. You can't look at a potential division and play fantasy matchmaker while ignoring historical trends which have few fighters jumping divisions.
The cruiserweight division, though, would get really exciting, really quickly. You get Cain Velasquez in there with most of his serious competition, and you have the opportunity to actually have lightheavyweight come up and challenge guys in the heavyweight division.
There's discussion about getting Jones or Machida or even Anderson Silva to fight a Frank Mir or Cheick Kongo. That only happens in the cruiserweight division. It never happens in the open waters of the heavyweight division where the smaller guy isn't guaranteed an opponent of predictable, reasonable size.
If its a one off fight I see little reason why a catchweight boute is not an acceptable alternative.