The turbos are the same size, wheels and housings. The 20 HP loss is at the top end. The cause is the A/R of the turbine housing. The auto has an A/R of around .58 and the 5 speed is .64. The auto spools up faster for good responce on the street where you don't have a choise of hanging in gears to keep the boost up like you can with a stick.
It's only 20hp but then there's also the extra loss in the auto trans too:-(