Yes, a chopped-wave transformer will help. The K-Line set transformer (larger one with the metal case) is one of the best for coaxing good performance out of those old motors. The sine wave has a sharp leading edge and amplitude is something like 36v peak-to-peak. You might notice a little more "buzz" compared to a traditional wound-field autotransformer.
Not sure about your expectations here... None of these postwar style Pullmor-motored steam locos will run at a constant slow speed for long distances, or for long periods of time. The gear ratios just aren't suited for this type of operation. Furthermore series-wound AC motors are VERY sensitive to changes in load. Get a level and make sure your mainline is absolutely pool-table flat, with NO rough track joints, and no pinch points in the gauge.
A couple of more radical options would be to replace the Pullmor with a Timko motor and some type of speed control, or retire your 8615 in favor of the Railking L&N Berk with Proto2, MTH p/n 30-1178-1.