I really like the layout of your list and the weight percentages for each of you (56% for you and 44% for Sibby). I get a little OCD with the gear list thing so I can appreciate your detail. Your 14 lb and 11 lb summer base weights are great, especially considering the weight of the packs and shelter. Candy & I just did a 3 day Sierra trip and our base weights were similar (11 lbs & 7 lbs, I carried the 'heavy' stuff).
I realize you already mentioned that the backpacks are not really up for debate. I have used Osprey packs and started this past summer with an Osprey Argon 85. It was amazingly comfortable with 45 lbs for my training hikes, but it weighed 6.2 lbs empty. Although I was really impressed with the hipbelt padding and suspension I ended up returning it as my base weight decreased.
The hipbelt on my Osprey pack was actually custom molded to my waist after being heated in an oven at REI. I thought it was the pinnacle of comfort.
After all that I have been amazed at how much more comfortable ultralight packs can be. My wife and I have been using Gossamer Gear packs. Candy has a Murmur (8 oz w/ hipbelt) and I use a Miniposa (14 oz) or a Gorilla (24 oz w/ internal frame). We returned Candy's Osprey Exos 46 as the Murmur was sooo much more comfortable. This is just my experience so I thought I would put it out there. . . if you ever consider a change.
Have you considered possibly using a lighter shelter? The Big Sky Evo 2P/2D/2V weighs only 48 oz. It's a 2 person double wall tent with 2 doors and 2 vestibules. The Gossamer Gear Squal Classic is a 2 person tent and weighs only 24 ozs. The Zpacks Hexamid Twin Tent is a 2 person tarp tent and weighs 10.5 ozs. A large Gossamer Gear polycro groundsheet weighs about 2.5 ozs and may be a good option instead of the 8.8 oz tent footprint. These options could save between 27.25 ozs to 65.25 ozs depending on which one you consider.
The other thing that has helped us save considerable weight is repackaging our misc items in small 1/4 oz nalgene bottles or dropper bottles. 1.4 oz of sunscreen, 1.76 oz for soap, 1.59 oz for toothpaste all adds up. I realize this may seem minor, but by repackaging and just bringing what you need for each trip you could save 3.5 ozs.
We started using Aqua Mira repackaged in small dropper bottles (1 oz total) and now leave the water filter and extra platypus bottle at home. This would save over 6 ozs on your current list.
Put all this together and you've chopped almost 5 pounds off your base weight. It really adds up quickly.
Check out this funny BPL article called Every Ounce Counts: http://www.backpackinglight.com/cgi-bin/backpackinglight/testimony_every_ounce_counts.html
I hope this is helpful. Have fun in Tasmania.