I have always done mine on the kettle. But, I have not owned my WMS very long. That might have something to do with it. When cooking pork loin or pork tenderloin either one, I normally put a rub on it and brown it all the way around directly over the coals for about 5 minutes per "side". Judging by the color, I might continue to cook it over the fire, rotating occasionally. After I have the outside the way I want it, I move it off the coals, put a temp probe in it and leave it until the desired temp. 140 is a bit too pink for me, but technically it's done. I normally bring it up to about 160.
I just read another thread here about cooking on the WSM without a pan. This might make a great cut of meat to try that with. If you are doing it on the WSM, maybe you could apply the same approximate technique on the WSM. I've never tried it. But, I imagine it would go about the same, albeit with times extended. I think might actually try that. I have a loin in the freezer. I might thaw it out and try it on the WSM this weekend.
I also like to slice mine as thin as I can. It makes great leftovers for sandwiches. One good thing is that you don't have to heat it as long to warm it up later.
Good luck and let us know what you do.