I believe I would conduct a small test to see if removing the spring would cause a grounding issue to show itself. Try running just the loco through your switches at the slowest speed it can manage. If the engine does in fact have poor grounding, it will likely stall. Then try the same experiment with the tender hooked on - I would even hook a couple more cars to the tender, just to keep tension on the drawbar. The idea, I believe, was that the tender wheels provided additional outside rail contact that could be conducted through the drawbar to keep things moving over switches. No expense to try it and see if there's any effect on your engine!