I believe that the CWC campaign is definitely different (and worse) under 1.91 then under 1.20. I am pretty certain because I was just re-doing the campaign under 1.91 (and it didn't seem to be as exciting as I remembered when I played it the first time using 1.30). BTW, I had the viewdistance at 900. So I uninstalled everything (I don't have many add-ons or other down-loads so it's no big deal for me), then I re-played under 1.20 (just straight original Operation Flashpoint). I only did the first two missions, but I can tell you that the first couple missions (Flashpoint and especially Combined Arms) are much more exciting under 1.20 than 1.91. I played them in veteran mode with all aids off, but not super-ai, in both 1.20 and 1.91. There's just a lot more going on under 1.20; a lot more enemy activity. I re-played Combined Arms a few times in both 1.20 and 1.91, so I'm pretty certain this is no fluke.
Is there any problem with having more than one installation? I'm tempted to have one at 1.20 for CWC, one at 1.30 for Red Hammer, one at 1.75 for Resistance, and one at 1.9x for down-loaded new missions. That way I can be sure as possible that I'm playing the missions under their design conditions.