The lowest I've had it in was around mid to upper 30's and I was very comfortable with just a light baselayer. I got it for $160 on sale last year around this time. I'm only 5' 6" and have plenty of room, but don't know how that would compare at 6' 3".
Here is the Marmot Ultra Elite 20 next to a Nalgene bottle for comparison. Not quite as small as down but for synthetic and weighing 2 lbs 5 oz in the compression sack Marmot provides it is pretty solid.
Don't know what temperature rating you are looking for but I've really liked the Marmot Ultra Elite 20 it packs down really small for a synthetic bag...