Start with the data points that are out there

A class catamarans. Newer Australian a cats are coming in at as low as 150 lbs (68 kg). These boats have gel coat finishes and foam cores and are made using resin infusion. They are rugged and don’t break under normal racing. In Bristol, RI which is a moderately windy venue there has not been a mast break in 2 years, this translates to ~ 25 boat years sailing without a mast break. Current A cats cost 17-23K new.

You can definitely build these boats lighter if you are willing to pay for it. Change to prepreg carbon with a nomex core autoclave cured hulls, cuben fiber mainsail, use of boron fiber in the mast, all composite rigging, lighter trampoline. The problem is that you now have a very expensive boat. What would it weigh? You could reduce the weight by at least 10% if you were willing to increase the boat cost to say 30-40K$. You are in the range of diminishing returns for a 60 kg boat.

On the other end of the spectrum is a modern C class catamaran. 25 ft long 16 ft wide, double trapeze, wing sail ~ 365 lbs (new Canadian boat) 165 kg. It is a very expensive toy. I don’t even want to guess at costs.

It seems to me if you want a light 2 person 20 footer that did not cost a huge sum, what I would go for is 8’6” beam (not 10) Resin infused, foam core hulls, carbon mast and glued carbon beams. It would be a unirig with a 33 ft wing (not pear) mast and a flat 240 ft^2 spinnaker. It would weigh about 250-275 lbs. Rigging would be dyform, attached with lashings and have aramid trap lines. More importantly, it would be easy to handle on the beach and awesome to sail.
It would not be all that much faster then an inter 20 in most conditions. It would cost around 30% more then an inter 20 and very few people would buy them. Those people that did buy them can’t understand why everyone else in the world does not send their I-20’s to the dumpster and get with the program. Soon Bob Curry pronounces that this new lightweight boat is just a fad.
The builder, in order to stay in business, cuts 2 feet of the bows of the boat, adds a jib, pours in an 80 lb bag of Portland cement in each hull, paints the masts silver and calls his boat an F-18. Soon he is outselling all the 20 footers in the world and making millions of dollars.
When the price of Portland cement increase to 90$ per barrel, he sells his boatbuilding business to an Arab named Vinny Marawck Al Saud and retires to Costa Rica where he buys a 3rd rate brothel and dies of a previously undiscovered and incurable venereal disease.

This is why light weight double handers will ultimately fail, and condoms are very important.

Eric Anderson