Manfred,
You have brought a really good topic here. You will find in all the games in the Third Wire series, aircrafts are not proprtional to their real world counterparts. For example, you will find an AI MiG-21 shoot down a F-22 in the game USING just GUN!!
Whilst most of the features of the game are historically accurate without modification, because the game actually emphasizes on close distance dogfighting, so close dogfighting capable aircrafts like MiG-21 stands a good chance to dedeat a once state-of-the-art fighter like the F-15 or F-16.
If you want your F-15 to be "backward compatible" with the the MiG-21, just change the INI file to make the MiG-21 invisble to you and yourAI wingmen.