Hi Paul,
I have the quartet of SP GP40Xs on my bench to do. I've noticed that the board that comes with the loco doesn't appear to have enough connections to seperate the lights for the full-meal deal SP lighting. I'm looking at using the Athearn board that comes with the Genesis locos instead, but it requires more wiring as there is no top motor strap on the bottom of the board as on the factory setup.
I like the idea of the slot for the rear truck wire.
I'm using Scale Sound Systems speakers which replace the rear weight, so I'll have to see what JT designed for that wire.
Your install looks good! Glad to know the RP module fits under the hood. I've often gotten everything hooked up and taped down, just to find out you need 1/16" more of room!!