I cannot comment on the ZW-L fully because I do not own one and had not been able to perform the same level of testing.
Lionel's design of both the GW180 and recent CW80 utilize only 2 FETs per channel, no inductors, and thus a lower through control path resistance. Also, then the source transformer is a 18-19V rather than a higher voltage.
Also, the Legacy powermaster circuit (which in theory is similar to the ZW-L) is also 2 FET based.
Thus, both in testing and design details, less internal resistance. How much, I cannot say, but again less than what you have now.