I dont think lensar would have a chance against barnett for many reasons...
His experience in fights
He has fough the best in the world and has done pretty well, only 5 losses.
He's beat ( Pedro Rizzo, Jeff Monson, Antonio R Nog, Alesksander Emeilianko, knocked out randy couture in 2nd rd, Mark Hunt via 1st rd sub) Lesnar doesnt know what top notch competition except when he lost to Mir, Couture just came back rusty.
Remember Barnett is a pro wrestler as well, and his striking is good. Brock will only have a about 10 lbs size diff on this guy. I think Lesnar has alot of potential but hes not ready for top end fighters yet... If mir looses this match it will be cus his hands. I think Lesnar will be better at take downs but Mir has the subs. So i think Lesnar's game plan will to be on the feet strikin and stuffin Mirs take down attempts