This is all very helpful and informative. I will do my own from scratch, except I have several engine harnesses to work with. Since mine's an R, I will have fewer wires.
Interesting that you chose to add-in TXL rather than GXL or even lesser wire, when you are mixing it with Subaru's inferior (?) wire. What was your logic in the decision?
I think the board is the way to go. You can have points at the front and back of the tunnel and at common intersections, so that you do not have to measure through the whole length of the car over and over.