Sleeping kit is where it is easiest to save volume and weight. If you succeed, you will not need front panniers and a front rack, saving 2-2.5 kg and at least 100 pounds. So use that money saved to get a better bag and mattress.
If you are cold, a warm mattress is the first thing. The Exped down mat is the warmest, but may be overkill. The Exped Synmat basic is still warm enough a bit below freezing, and much cheaper and lighter. The lightest and most compact mattress for someone who sleeps cold is the Thermarest Neoair AllSeason at 540 grams. It packs very small.
As for sleeping bags I think the choice is in ascending order of quality and price: Alpkit Skyehigh, Alpkit Pidedream and PHD. Each with the down weight that matches the temperature range that you need. Normally PHD bags are very expensive, but their sale prices are much better (there is a sale coming up soon). They are also pretty narrow, so beware if you are larger.
This is all for solo use. For two people I think topbags for two are the way to go.
Willem