I run a 42RH overdrive trans with converter lockup in my Dart and can tell you that when the converter locks up the RPM's drop 300 rpm's pretty much on the nose.
I can also tell you that when it is in lockup it feels just like a stick in high gear, as it has that same direct drive feel through the drivetrain.
In the summer I also notice a drop even in the engine temps when in lockup since that is where they cool best as already mentioned.