Your gear list is pretty good! Here are my suggestions, in no particular order.
- Ditch the bivy sack - that's what a tent is for!
- Get a compression stuff sack for your sleeping bag. The uncompressed stuff sack should be about the same size as your current stuff sack. It makes a big difference to free up space inside your pack.
- Smaller things pack easier than bigger, so I use two 500mL nalgene bottles instead of one big 1L bottle. I don't carry more than that, but I make sure to drink 1L in the morning and at least 2L in the evening (more if you include soup). One bottle also doubles as my mug. This way I can make a hot drink, close the lid, and stuff it in my jacket while I finish cooking.
- Loose fabric stuffs into the pack better than jammed stuff sacks, so get rid of any stuff sacks for your clothes. That way you can take advantage of all the free space to stuff things into. This advice includes the tent.
- Speaking of the tent, don't bring anchor material like snow stakes or tent pegs. You can use ski poles, skis, and shovels instead. Make sure you have guy-lines at every conceivable point, including the four corners.
- I use a 42L pack that fits everything. Read about it here, Gear Review: Black Diamond Sphynx 42L pack. Its all the way at the bottom.
- I don't use booties, but I understand why everyone else does.
- Get a bowl from Fozzils. These things are great for packing since the open flat, and are better than the competitors because they use snaps to hold shape. They're a little stiff at first, so make sure you warm up the plastic and bed a couple of times before the trip.
- My sleeping pad is rolled and strapped to the outside to save space, otherwise I pack everything inside.
- I may have missed it, but each of you also need a shovel and probe. Otherwise that beacon will just be useful for "X-marks-my-bro's-body." The shovel also doubles really well for tent anchoring, platform building, and snow-for-water gathering.
- And in case of emergencies, make sure you have ONE 1st Aid Kit and ONE Repair Kit for the group. If you're using an MSR liquid fuel stove, carry a full spare pump instead of just parts - it really sucks to have to fix a pump at the end of a long day before you can eat. There's no point in carrying a full binding kit if you have no way to mount it - think about what you're carrying and why.
- Headlamps are essential too. Be clever and buy one that uses the same batteries as your beacon. Now you only need one set of spare AAA's!