It's often said that boats are sold by the pound; epoxy, glass, aluminum, stainless, and the labor to put it all together. Multihulls run higher than monohulls because the keel is cheap in relation, they don't need 2 of so many things, and because the construction is less convoluted. I'll stick to multihulls, since it's what I know.
For the sake of argument, let's say you can buy a Gemini Legacy for $275,000/9,400 pounds, or about $30/pound. A performance boat like the Stiletto 27--if one could be bought new-- would be about $80,000/1,250 pounds, or about $65/pound. Gunboats run about $80/pound. Ouch. Used cars, on the other hand, run perhaps $2-$4/pound; though they don't float so well, they do fly to windward.
What would we pay for a lighter boat with better performance, strength being the same? Would we pay $40/pound? Most of us bought used--I paid about $13/pound--but I'd spend a little for speed, maybe $20/pound. By that logic, we would pay ~ $30/pound for every bit we could save on our current boat, whether from taking out the trash. Hauling less fuel when practical, or pumping the holding tank more often. We would evaluate every project, looking for places to swap composites for steel and Spectra for stainless steel cable.
Anchors. A 35-pound new-age anchor seem to be the thing if you want to stay in one place. Certainly the second anchor should be a Fortress? A $239 Fortress F-16 compares with a Danforth 16 for $89, but at 6 pounds more. $25/pound for the savings. And of course the Fortress is much easier to handle, so very OK; doctor bills for my back add up.
Chain. Proof coil or BBB vs. G4. For a given rating, G4 is cheaper. Perhaps it is less durable in the very long run, but given the value we place on weight savings, an easy call.
Chain. G4 vs rope. Again, rope is lighter and cheaper. But rope doesn't feed through the windlass as well and cuts are possible. I chose 100 feet of chain backed by rope as a compromise. In the shallow waters of the Chesapeake that puts me on all-chain 98 % of the time. All-chain for a multihull is hard to defend, and I would rather have some rope for shock absorption in extreme conditions. A dollar per pound foolish compromise? The windlass made the decision for me.
Lifelines. Stainless is about 3 times heavier and 70% more expensive than Amsteel, even when the Amsteel is sized to be 3 times stronger. Only $12/pound for the savings and a $120 savings for my boat. Durability seems good, as the first reports trickle in. I've seen corroded stainless snap without warning. I'm going with 3/16-inch Amsteel in a few years. Should be good for a 0.02 knot improvement (yes, I calculated that figure).
Locker stuff. Always a hard call. It's always hard to say which spares and what toys will save the day, but at $20/pound to haul it around we need to be circumspect. Certainly seasonal items such as quilts and wet suits can go home for dry storage. How much is the labor? Heck, they need cleaned or serviced anyway.
Fuel vent filters. A 1-pound vent filter eliminates the need to keep a full tank to reduce breathing, perhaps reducing by 100-150 pounds the amount of fuel that must be carried day sailing and on short trips. Only about $1/pound. A bargain. They also reduce evaporative fuel loss, making them free in the long run.
Down-sizing ropes. So long as the stretch and handling characteristics of the high-modulus lines are suitable, makes sense even for the non-racer. But sometimes the handling and stretch characteristics are wrong--I like some stretch in the traveler and topping lift. I like lines with some size, nothing below 3/8-inch. I put in a free Spectra traveler line and I don't like it better. I have Spectra genoa sheets an they are fantastic. I would decide based upon function, not weight.
Leaks. Are any of the crash tanks holding water? A hundred pounds of water would create a $3000 loss in value, one more reason to get it fixed.
Aluminum or composite propane tanks. A good weight savings (~ $15/pound) and non-corroding. I'll go that way if mine get shabby, but I keep them painted for now.
Holding tank. Pumping more often couldn't hurt, but I'm lazy. A Raritan Lectrasan might make sense for some folks--it weighs about the same as 10 gallons of water when full--but that is about what I average in my tank and I prefer the practice of shore-side pumping-outs. Additionally, my home marina and local sailing area is a no-discharge-zone. No weight savings for smaller boats or smaller families, but worth considering for larger boats.
Freshwater tank. There's really no reason for it to be full all of the time. But I never know when I'm going to head out for a day or two.
Batteries. There may be some real opportunity in the near future, when laptop tech (lithium) approaches lead/acid for economy. We'll need to look at pounds/amp-hour and cycles.
Projects. Yep, I've added a few pounds here and there. But I use aluminum and light ply/composite constructions where ever practical, instead of stainless or solid wood or glass. The solar panel mount was aluminum and minimal. With the exception of the sea chest (composites would have been all wrong there) most of my shelf and box projects are 1/4-inch ply, glass and epoxy. I use fabric in place of wood where possible. Lashings and stropes save ounces.
So what's on my lighten-up list? A Mast-Mate mast ladder; haven't used it in years, will do a quick inspection climb and take it home. I keep accenders on-board anyway, so in an emergency they will serve. My bike rack can stay at home with the bike. I've got some spare blocks that could be at home. I've got a little gut to lose before ice climbing season. What stays? Any part I've used before: sheer pins, spare props, oar locks, cabinet catches, engine parts, electrical stuff, and some bits of rope (2 extra rodes and some small stuff). Food and beer.
Weight saving projects? There is certainly a noticeable difference, loaded for cruising vs. empty tanks and no crew. I remember the sea trial on the day I sold my Stiletto; emptied of superfluous stuff and with a clean bottom she flew, reaching at well over wind speed, into the high teens under working sail alone. I wanted to keep her.