It's beem discussed here in the past guys would take old 512files and stack the rom twice and it would run on a 1mb flash chip on the b5 s4. Yes prj MOST are NOT interchangintereable but you are wrong in this case
Try it the other way around and see. Have you actually done this or are you talking out of your ass?
Yes, you can stack 551C twice into a newer ECU, but the newer ECU versions have some calls to functions which are not present in 551C bootrom...
I know for a fact 551K won't run on it, because I actually tested it, and if you read the two roms with minimon you can see that some code is missing.