A Kelvin Jacket or Kelvin Vest is wonderful for layering under a 90% jacket for warmth. They are very light to carry and stuff into small spaces. They also make great pillows when placed in a stuff sack. For colder weather I would recommend the Traverse underwear. I also like merino wool underwear. The 90% pants are great. Those were the only pants I wore on my moose hunt in the Yukon with long underwear beneath the pants. For rain get any of the lightweight, breathable membrane jackets and pants from North Face, Mountain Hardware, Marmot, etc. They are light and fold into small spaces. The material is somewhat noisy, but who cares when it is raining. I use the Ex Officio boxers too, and I know what you are saying!
I love my Sitka Gear. I have three of their packs, three pieces of headgear, all of their long underwear including extras, the Ascent pants, 90% pants, 3 of the 90% jackets, two sets of the Celcius jackets and bibs, 3 Celcius vests, the Kelvin jacket and vest, and probably something I'm not thinking of. I also bought a bunch more for my son. I hate to think of how much money I have invested in Sitka Gear products, but it is way more than I initially thought I ever would. I got extra Sitka clothing so I could leave some of it where I do a lot of hunting and not have to haul it around every time, but still have the same clothing at home when I go elsewhere. If I had to limit myself to two pieces of the Sitka Gear clothing it would be the 90% jacket and pants because they really are that versatile. A Primaloft jacket, whether it is the Sitka Kelvin or some other brand, turns that outfit into something that works in colder weather. I don't have the Kelvin pants, but I bet they would work well too for those who sit in treestands a lot.
There is other good clothing on the market that is much less expensive than Sitka Gear. I just think the Sitka Gear is the best for many uses (but not everything).
Allan