@kortina: you probably mean the AGA core, as the Minimig Amiga core is not yet 15 Khz-capable AFAIK.
All I can say is that I have a 72 cm 4:3 Panasonic CRT TV that I bought in 2004, and the AGA core picture running an old Amiga game looks to me exactly like the one that my Amiga 1000 produces on this TV via SCART cable. Not all old games run on AGA (or on the AGA core respectively), but Wings of Fury does, and I wouldn't know whether this is MIST or a real Amiga that I'm playing it on.
I never had an AGA Amiga, so I can't judge the picture of AGA games.
Have you tried it on a different TV yet? Try to get a 4:3 CRT TV as well, as long as they are still widely available almost for free. I hope you have the required space at home.