The drivetrain in this loco is based on a postwar 736. I believe the gear ratio is only 8.33 to 1. The Timko motor is small and probably makes less torque than the original slow-turning Pullmor motor did. Search the forum for another fellow who upgraded his Berk to a Timko motor- despite his hopes, it didn't run as well as "modern" locos with better (numerically higher) gearing.
All of these Berks and northerns have the worm wheel on the last driving axle. This is a sub-optimal design. For best performance and the least amount of "slop" in the side rods, the worm wheel should have been on one of the middle axles.
If you must have a traditionally-sized 'J', you might be happier with the RailKing model (14:1 gear ratio) With PS2 or ERR cruise, it will run better than the Warhorse and you could weather it yourself. A Railking Mohawk (17:1) or a LionChief Plus Mikado (20:1) would run better still. Ironically, the Mikado also has its worm wheel on the 4th axle. 
A better mouse trap, but I guess there's always room for improvement!
