I got a 52-12 4.5/3.5mm the other day and wrote something on the review thread, if any help?
Tried it yesterday again and it was a windy and chilly day on a onshore mush southcoast. I was 40% convinced that 52-12 suit (with techno butter cup cake) was the wrong choice for the condition but surprisingly kept me warm enough for 2hrs in the water. Just warm enough, bit warmer than my C-skin 3/4/5mm and less flushing through neck (hardly any flushing from wrist and ankle, very good seal). I wouldn't say it was as "super warm" as the guy with only one post said. Still not convinced that it'll keep me warm in the coldest winter days. But again, it kept me warm enough, less flushing, and alot more flexibility (due to thinner lining) than my winter wetsuit.
I have never owned Xcel so hard to comment on the comparison. But I guess if you are after guaranteed warmth, then Xcel may be the safer option, as you may know. If you value flexibility more and can sacrifice bit of warmth, then might be worth a try on O'neill.
I hope this helps.
PS I know a guy who's taller and bigger than me and looking for a hooded suit; I forwarded your link about your R3, I hope you don't mind!