Just realized that I forgot to include the switching current in my calculations. If I subtract that form the input current, I get a better IN/OUT ratio. I recalculated my data and found out that instead of 24% I am getting back 30% form my core. It is still too little, but this is just for information.
Anyway, I got my hands on a bigger induction motor core. I took it out form an old Russian 180W AC motor, that I found laying around. The plates are not isolated form each other, you can see four welding stitches on each side of the core. I think, that the hardest part will be the wiring of this core, I will this and see how it goes. If I manage to do this, the rest should be easy