Even if, and I'd put a big if, he's just a genetic freak that achieves 285lbs with no sort of gear, ever, he's still fighting at 285 against 230, 245lb guys.
He's a great wrestler, yes, but he uses his brute strength a lot more than fighting skills. Same thing I mentioned about BJ and GSP. BJ is a great at takedown D, and a black belt in Jiu Jitsu. Yet, GSP just dominated him. Do I think GSP is a better fighter than BJ? Yes. Do I think having more skills was the only thing that dictated that fight? No. The 20, 30lbs difference between both fighters played a HUGE role there as well. Hence why they are in different weight classes.
When a fighter has 50lbs on another it's never going to be a fair fight. Do I blame Lesnar for being huge? No. But I don't think it's fair to have him go against someone who may be just 210lbs (just as an example). He's going to have a HUGE edge. Lesnar is not one to blame here, the UFC is for allowing this huge disparity between fighters. And that's why they wouldn't create a 225lb class, because if they did noone would give a damn about 225+ as it would be pretty much a weight class of Lesnar and... Lesnar...
And ok, I didn't know he was 285 at college, but heck, just look at football players. Some of them more than obviously take gear and never get caught. One that has recently brought up those "rumors" is Cushing from USC.
Check it out
All I'm saying is, weight difference is a huge factor, and fighting smaller fighters is a huge edge already.