Jeff:
I have owned all the Leica 180mm R lenses. In the end I settled for the 180mm APO Summicron and the 180mm f3.4 APO Telyt.
I shot a comparison of the 180mm f2.8 APO versus the 180mm f3.4 APO and the 2.8 APO was slightly better, especially when used with the extenders, but there was very little advantage to the 2.8 APO if you were shooting both lenses at f5.6 or f8 and without extenders. Pretty well as described by Mr. Puts. At the time I think I paid $500 for the f3.4 and about $2,500 for the 2.8 APO. I decided if I really didn't need three 180mm APO and settled for the 180mm f3.4 when I wanted a light travel lens, the 180mm APO Summicron, when I needed the speed. The 180mm f2.8 APO was sold.
Incidentally, my preference if weight wasn't a concern was to shoot with the 105-280mm when I need a telephoto in the 180mm range. It was also almost as good as the APO lenses, plus combined with the 19mm, 21-35mm, 35-70mm f2.8, I had pretty well everything covered for scenic photos. The 180mm f3.4 would be packed for when I didn't want the weight of the 105-280mm, but still wanted a bit of reach.
Sadly, when I sold the DMR and went to Nikon, I couldn't justify adapting these lenses, as they were all pretty high value lenses and not worth the risk. Some went to members here for use on the new Leica M, and a couple of the more rare lenses went to cinematographers to use on RED cameras.