I shoot Production with a 17 and CO with a milled 17, I have shot limited as well.
I really enjoy CO, the better guys locally are right on the heels of, and sometimes beat M and GM Open shooters in the Overall which of course doesn't matter, but it gives you an idea the level of talent at the top. There isn't nearly the participation of Limited, the only sort of annoying thing for me is a whole lot of gear questions from people. I also find the dot made dry fire a whole lot more honest and the aggressive manner of running stages carried over pretty well to production.
MOS is definitely production legal will the cover plate, milled I think is with a cover plate, not sure about without.
I use a 6moa Viper, but the DPP 2.5 is easily the most popular, I haven't looked through one, everyone says it blooms larger cranked up, I know I wouldn't want anything smaller than 6moa.
Limited, there's a whole lot of competition, 40 is kinda a p.i.t.a. for me to reload, Major is sort of interesting, I dunno, it just doesn't appeal to me nearly as much.