It's easy to get overwhelmed with the amount of shit out there. Focus on a pack, sleeping bag and a mat. One at a time. Then maybe get a tent, 4 1/4lbs isn't that bad for two people, especially if you're rocking a larger tent for comfort.

How much am I sacrificing in comfort and durability by dropping 5-10 pounds off a traditional lightweight backpacking load?

Everybody has different levels of comfort. I use UL to cut weight in unnecessary areas of my kit. While maintaining the comfort I enjoy from other parts of my kit. So I can take binoculars with me, decks of cards and a slightly roomier/heavier tent so I don't feel miserable inside it.

You can cut 4-5lbs off your pack weight without sacrificing practicality or durability much. You could go further and cut 7-8lbs off pack weight if you cut into practicality and sacrifice durability.

Most modern backpacking-specific packs probably have an average weight around 4lbs. There are some Kifaru packs around that weight if you want to stay with a tactical looking pack(14r and Woodsman prob, there's a larger woodsman pack that can get up to like 80liters I think). I'm not as familiar with Mystery Ranch like you have, but a 10lb pack is just outrageous for your basic backpacking trips imo.

If you got a 4-5lb pack. A lighter sleeping bag, and a thermarest pad or something that would cut out close to 8lbs on the spot. Most backpacking sleeping bags can weigh around 2lbs and still be warm as all get out.