Currently have a synthetic sleeping bag that is 4.1 lbs. mid range Binos (Steiner), and a cheap pack (but functional). At this time I am able to upgrade one (high end used from the classifieds). How would you prioritize them?
Sleeping bag gets my vote. The other 2 you may find a great deal on used items that are in perfect order. The bag is not an item I would want to buy used and as you get close to summer there will be some great sales.
I would get the new pack setup. The bag is only a burden in and out. The pack will hopefully be making numerous heavy trips. A nice pack compared to what you have now will lessen the "suck" of carrying some less than ideal heavy and bulky gear.
That's great you've weighed your gear to upgrade. I did the same thing several years ago to figure out the items that weighed the most that would make the most sense to replace. I have a feeling you can upgrade your bag by around 1+ lb. I'm not sure how much your current pack weighs? When I upgraded my pack I lost around 3 lbs of weight and the Kifaru pack/frame I bought was a giant upgrade in quality.
If you do much backpack hunting and packing of game a quality bag/frame is worth it's weight in gold! I glass for hours and hours each day. Quality binos and spotting scope are also top priority for me. One thing I did when upgrading was sell a bunch of things I didn't use around the house and items I was upgrading to off-set the expense. I also spent time finding great sales and almost new items on Craigslist, Ebay, and this website's classified sections. As examples, I found a great deal on demo Leica binos and Swaro spotter on this website's classified section that were about 1/2 price.
As someone else still doing the ole upgrade shuffle, I'd say:
Pack, Bag, Binos
Depends a little on other factors (how much does your pack weigh, how does it do at heavy weights, what is your target budget for upgrades this year). With packs, there are other well regarded brands that are surprisingly affordable (Mystery Ranch, Seek Outside). It amazes me how cheap the MR packs specifically run on sale compared to how well they are viewed. If you get a MR pack for $300, maybe that leaves room to upgrade to a quality, 2.5# sleeping bag or 1.5# quilt as well, rather than shelling $600+ for a pack setup alone.