If you are carrying 40 lbs, the extra ~3-4 lbs that your current pack weighs really isnt the issue , is it?
It is easier to carry lightwt gear in a conventional pack, than to try to carry heavy gear in a lightwt pack.
2-3 days isnt much food, so you really dont need a large pack for that if your gear is compact.
UL packs arent the same as the pack you are used too. They are not rigid, and thickly padded. Your mindset of what a pack is, and needs to be, will have to change. However, if you keep your gear LIGHT, they will take you to a new level. Learning how to pack a pack best with YOUR gear is critical too, you dont just throw everything in
I like to say that there is more of a difference in backpacking experience between 15-20 lbs, than any other wt. 20, is still like carrying a pack. 15, is like not at all. My target is always to stay under 20 if possible, fully loaded with 2L water.
Yours may vary, but I would say to not PLAN to have a pack wt ever >27 lbs with a lightwt pack. A few will carry 30-35 OK for short periods, but you are possibly going to be better off with a heavier pack if that is your norm. Fitting stuff in is also going to be a problem.
A baseweight of 18-20 lbs, is not particularly lightwt. You need to drop at least 5 lbs from that IMO before you start considering UL packs,if you want room for food and water. 3 days food is 5 lbs, and 2L water is 4.4. 20lb base puts you at 30 lbs, minimum.
If you post a gear list, you will get suggestions on what to cut out and what to replace. The big 4, shelter, sleeping bag , sleeping pd, pack, are your most critical. These need to total about 8 lbs max. Next you probably need to keep clothing and insulation layers down to about 2 lb total. Raingear no more than 1 lb. The rest is ez, leave whats not needed home.
Order and try packs from ULA, granite gear, gossamer gear. If you are really tending toward that high wt, ULA Catalyst is probably the best pack. If you can get it down by 5 lbs, the ULA circuit or granite gear crown VC 60.