Well, 45 pounds, maximum load, is a lot of weight. I personally haven't carried that kind of weight in about 30 years. Even on my long unsipported trips my maximum loads are always less than 30pounds for 3 weeks out.
I usually carry an 8 or 12oz bottle of scotch. One of those luxuries I really enjoy at night is a cup of hot cocoa with a shot of scotch. Especially when rain is closing in. Anyway, theoreticallly, light weight starts at 10-20 pounds of base weight. With a 15 pound base load, you are certainly looking at lightweight. Regardless of food, fuel and water.
Are you carrying food for everyone? 15 lbs of sausage, 4 lbs of tortillas, 2 lbs of cheese and 3 lbs of miscellaneous junk food? For 5 people? An easy way to figure what is OK for packing weights is to figure about 2 to 2.5 pounds per day per person for food. I usually carry between 1.2 and 1.75 pounds per day. On long trips, I pare this down to 1.2 pounds per day. But, my older metabolism only needs about 2500C per day. I might loose a few pounds, I *can* afford it. One way to cut down food is to package food in daily baggies. After a few outings, you will know if you took enough. If you carry food out, that is a huge waste. Better to be slightly short than to carry food out. I finished a two week trip out with 2 enrgy bars and a candy bar. That would have been lunch, but I was picked up at 1300.
Water, there is not a lot you can do. Depending on your exact conditions, you will need between 1/2 and 2 gallons(or more) per day. You can get buy on 2 cups per day for a day or two, but you will be thirsty. Alcohol will act like a diuretic, flushing water out of you. So, if you drink a lot, you will need more. You cannot controll this, it is one of the side effects of enjoying an evening. If there is none on the trail, then you *have* to carry it. It doesn't matter if you carry fresh fruit, veggies and other "wet" foods, you need the water.
The good thing is that water is fairly dense. A few 1 gallon platty style bags will squish around pretty well in your pack. It won't take up a lot of space for it's weight. Mark every watering hole on you map, though usually trails will go from one to the next. Get current conditions and status reports about water. You only need water between watering holes. It doesn't make sense to carry it from one to another. I plan on 2 liters for each camp, one at night, one in the morning. And another for on the trail. But will vary by the person and the environment. This is minimum, not usefull for deserts or hot weather.
For ultralight packing, there are several that will fit the bill. I use a Gossamer Gear MiniPosa for longer trips. The Gorilla is about the same size. Both will cary 30 pounds. A little more when you start. Shoulder straps can be anoying with heavy loads, and the MiniPosa has better straps. But, this is obsolete. The Mariposa is bigger, but will not handle much heavier loads. All ultralight gear will be a trade off in durability and weight. Generally speaking, with a 30 pound load, I would look at a internal framed pack. Most external frames will add about 8-12 ouces in frame. Good for support, bad for weight.
You have to take care of UL gear. You cannot drop it, you set it down. You pick a relativly clear spot to set it on. Even then, you can plan on a few repairs over ten years. Nope, you cannot sit on it, or drag it up mountains. Some of the heavier packs might work better. The Z55 you mention is heavy. It is well over 3.5 pounds. The gorilla is more like 22oz or 1#8 or so (depending on options) or less than half that weight. But, it is not as durable. Your Z55 will be around long after the Gorilla has died. You will always carry the extra weight for that durability. The ULA Circuit will handle 30 pounds and is fairly durable. (You can overload it at the start of a trip by 5 pounds or so.) SMD, Granite Gear, and ZPacks make some that will handle 30 pounds. None will be as durable as the Gregory, though. All will save you a 1.5-2 pounds of weight, depending on the size.
So, getting back to your origonal question, yes, you can certainly use a UL pack. There is a large variety of sub 2-pound (32oz) packs out there. Look for one with some sort of internal frame.