Long time, but I have an Ilsa 21 cm, which is about 3 mm total thick and has tiny legs to keep it slightly off the cooking surface, reducing the risk of overheating the appliance. That 3 mm is well enough to notice an increase in evennes and it doesn’t take so long to heat up.
It makes it so that I can for example pretty much guarantee rather evenly browned and crispy edge swedish pancakes with my Skeppshult 23cm cast iron pancake iron, with a cooking surface about 21,5 cm. Without it, depending on preheat, they could have lighter edge areas if I make a full surface pancake, and then start to get better to good from the second or third one or so.
Then I have a thicker and larger Kinghoff 6 mm, 23,5cm, this one enables me to use my 28 cm Skeppshult frypan with the pancakes. It really does improve the evennes quite a bit indeed, but it doesn’t have the tiny legs.
Here’s a photo with both the adapters in use for the mentioned pans. The back hobs are smaller than the front left, but the Ilsa is enough there also, without it there, no way to cook one like that, but on the front burner the wooden handle pan can do “pretty good” without it also as mentioned…
I’m not saying I’d be a huge fan of using the adapters though… they make it more fiddly and I often make my swedish pancakes etc without them also. Since the adapters though, I am using them every time I pan fry for example fish fillets with my cast iron, for this purpose they are a very good help imo.
I’m not sure which one I like better if I am doing more general cooking with the 28 cm pan. The thinner and legged Ilsa appears faster in use and has seemed able to replenish heat faster, with “good enough” improvement in evennes. The Ilsa is now 1,5 years old and has not warped or anything.
Also not sure if the Kinghoff has a few times made my cooktop cycle off heat for a moment, but it made things things seem a bit slower on those occasions with bigger piles of food…