I've skied the R:ex's and didn't find them particularly light. Not a bad ski but bit great either. For my use I ended up with Axis XP. Similar dimension to the R:Ex but livlier and a much higher speed limit. The XP is an alpine board for me and my old XScreams (187's) are getting a remount with a set of Diamir's. I expect that to be a pretty good bc ski. Really easy ski to work, very quick and light. Good characteristics when picking your way through the gnarly. They come with a speed limit though and that isn't a great characteristic when ripping up a bowl ... At 68mm under foot they are the narrow end of the mid-fat. I'm 205 to 210 lbs. I suspect I'll fight that 68 mm width. At 165 lbs you would do a lot better in the float dept that I will. (I gotta loose the gut, it will all get easier then, sigh ...) The REx's and XP's or B2's 78mm to 80mm waste is floatier and allows for a shorter length - or to go like blazes on a longer length.
It is probably common knowledge here but I was suprised to find the 181 XP almost exactly the same contact length as the 187 Solly. I had intened to buy a shorter length and forgot about the euro vs us measurment method differences. As it turns out, I think the 181 was a good length for me. Those ski's just rip! Too heavey, at least with the 912 TI on plate binding setup for AT IMHO. It does go like to go fast though!
I also have a 190 AK Launcher in the quiver. that is not an appropriate ski for AT. A very cool ride in the right conditions but that pup is heavy, heavy, heavy! It would kill you to lug that thing up hill. Strictly a lift/cat/heli board but a classic.
My .02 and worth every penny!