Regarding smoothy, I think the most anyone can do is speak from personal experience. My personal experience is that on two consecutive drylock 5/4's, the smoothy seemed to work well, but was also the weakest part of the suit. Both of those suits started failing because at least xcel glues those panels in place, so once the glue starts to fail, you end up with separating seams.
I'm surfing in New England, so I'm dealing with 38-42 degree water in the winter and typically surfing during or post-storms with wind and air as low as 15-20 degrees (some people go out when its colder, thats about where I draw the line). My Feral 6/5/4 with no smoothy has worked just as good, if not better, in cold and windy weather this winter. There was at least one session where I was out and with the wind chill it was between 0-10 degrees Fahrenheit and I was out for at least 2 hours.
Maybe smoothy is more critical on suits <5mm or 6mm, maybe the xcel smoothy being so vulnerable isn't replicated on other suits (though as mentioned above, the Japanese Neoprene smoothy is very susceptible to fingernail nicks and such). Maybe a yamamoto suit with smoothy would be even warmer (I know there are some on the market that include it). I don't know, I used to think it was super important, but I've been fine without it.