The USGI system will certainly keep you toasty warm in any American temperatures. It's pretty heavy, but I like it. I dedicate about 15lbs of my weight to shelter, anyways, and with the USGI system, it's really all the shelter you need.
That being said, if you know and have time for a little bushcraft, you can snuggle up significantly closer to the fire with a wool blanket and it will keep you just as warm. I ditched the USGI because a wool blanket is more durable, insulates while wet, and I can get REALLY close to the fire with it (all qualities I need), but that doesn't make the USGI a horrible sleep system. Sure, it was designed, built, and manufactured by the lowest bidder, but it still had to meet operational requirements! :thumb: The USGI is also great in that it is modular, so you can mix and match for the seasons, which sounds like it would work well for you.
As far as a sleeping mat goes, I'm looking into buying more Thermarest products after playing with this Thermarest prolite 3 I found lying around. Foam pads insulate well (I think even better than the thermarest), but they're large to pack around, and, worst of all, they create pressure points (for me, I'm 300 lbs) - something you want to avoid while sleeping on the ground. My thermarest packs smaller (but slightly heavier), has enough insulation, and, most importantly, sleeps without pressure points!
If you don't value insulating while wet, durability, and fire resistance, then ditch the wool blanket and just use pieces of the modular sleeping system. Wool blankets are just too heavy and bulky if you're not looking for those qualities in a sleep system. :thumb: