Comfort of Kofod Larsen sofas?  

I think there are too many variables to really answer the question, which depend on the type of upholstery fabric/vinyl/leather used, the density of the foam that is in the cushions, and the condition of the elastic straps that support the cushions. As well as the fitting job that the upholsterer did when sizing the cushions.
I have on many occasions been able to sit in Kofod Larsen and other wood frames sofas/chairs both before and after they have been reupholstered, and the difference can be like night and day. If the work is done properly, they can be very comfortable.

What tchp said. I've made cushions for many wood frame chairs like this, and a few sofas, and after doing the webbing I always tested various densities of foam before making the cushions. There is always one density that feels just right, and the back should always be at least one level less dense than the seat or it will feel too hard (or the seat will feel too soft). This is because most of your weight is on the seat cushion.
The cushions should fit snugly inside the frames with a little indent to them on the sides where they meet the posts---but not a lot. If they are a little too small, they will get out of whack in use. But the weight is also a factor---a 3" thick back cushion in a medium or soft density does not weigh a whole lot and won't always settle back into place when a person gets up from the chair.
If you want ultra comfort plus weightier cushions, then go for latex foam. It is more resilient than urethane foam and will not develop hollows with use, which is an unavoidable characteristic of urethane foam. Cushions should last 20 years or so---double or triple that of urethane foam. Even at 3" thick it's pretty heavy so it will definitely stay put---plus it's pretty great in general for comfort. BUT it costs 3-4 times as much and is not readily available--there are a few places that sell it online and you can buy it locally if you're near one of them but that's about it. Well, no---you could also buy mattress toppers on ebay or other sites but that's a bit hit or miss when it comes to gauging the density. Plus they are usually between 1" and 3" thick so you have to layer them to get the right thickness for the seat cushion--kind of a pain in the neck.
The other factor is the fabric. Some fabrics slip around more, some will tend to grip the webbing better. You can also have your upholsterer cover the webbing with a heavy knit fabric that is stapled to the frame. This was done in some old Danish furniture; it will provide a smoother substrate than the webbing for the fabric, resulting in less wear over the years.
Get a good, durable Danish wool like those made by Kvadrat (Maharam is the US distributor, or you can find mill ends on modern-fabrics.com and ebay). It will be the most appropriate style-wise for the furniture and will look great, too!

Latex foam is what the catalogs say was originally used, and it would show less tendency to get out of place. If you sheet a sheet of 1in. latex foam on a table and tug on the edge, the edge just moves around and stays there. If you do the same with a sheet of latex foam the entire sheet moves. (Except for memory foam, with is closer to latex). This is why urethane foam cushions, especially when cut too small moves around. You sit on it, and then the whole cushion jumps into a new location.