I was never satisfied with the Baja Designs rear taillight when I had my XR. It let in water and dirt, so as to make the light a lot less visible. Maybe you could see about sealing it up to prevent that.
Also, some thought may be given to the LED insert that replaces the stock taillight bulb. It's real bright and has a brake light function as well. DRC makes it and it's available from Wheeling Cycle Supply. Other places too, I'd imagine.
You have a very nice build there and it gives me thought for my new to me XRR.