Artest doesn't have the lateral quickness he used to but he is still a top tier defender in this league. Still defended Lebron James better than anyone else in the league did. What he lost in speed he's gained in muscle and defensive IQ. Maybe he won't be able to chase every quick SG but he excels at defending bigger SFs like Pierce, Melo and Bron.
The scary thing about this Lakers team is they don't even really need Artest's offense. They are going to turn him loose on the defensive end much like the Spurs use to do with Bowen.
So maybe Manu beats Artest off the dribble a few teams. He'll still be running into two, long 7 footers in the paint. A combination of Pau/Bynum, Odom/Pau, Bynum/Odom will be on the floor most of the time. And we haven't talked about Kobe playing the free safety role (which he's great at) since he doesn't have to chase around the other team's best offensive option anymore.