If you add weight then the model is likely to go slower anyway!
It may be that you are attempting to drive the model faster than it was designed for. The Scnellboots were fast but they did not have planing hulls so they would natuarlly roll in a tight turn unless speed was reduced.
A quick Google brings up the following:
Small rudders added on either side of the main rudder could be angled outboard to 30 degrees, creating at high speed what is known as the Lürssen Effect.[5] This drew in an "air pocket slightly behind the three propellers, increasing their efficiency, reducing the stern wave and keeping the boat at a nearly horizontal attitude".[6] This was an important innovation as the horizontal attitude lifted the stern, allowing even greater speed, and the reduced stern wave made E-boats harder to see, especially at night.
Colin