There’s a good chance you won’t get any responses to your question. Ideally, to get objective views you’d need someone who currently owns both a mk5 and mk7.5 GTI (a 2018 car will be a mk7.5, not a mk7) who can compare the two back to back, and I dare say there aren’t many who currently own both cars.
There’ll no doubt be some forum members with a mk7.5 GTI who owned a mk5 GTI in the past. However, remembering the finer points of the performance of a mk5 GTI’s sound system from a number of years ago and being able to make an objective like for like comparison with a mk7.5’s system wouldn’t be easy IMHO (memories fade, mk5 GTI sound systems may have been been fitted with aftermarket head units and / or speakers). I owned a 2007 mk5 GT which had the optional RCD500 unit (multi CD changer). I can remember that it was fine for a 2007 system and it did the job, but I wouldn’t be able to comment objectively on how the sound quality compared with the infotainment unit in my current car as it’s 11 years ago since I owned the mk5.
I believe forum member @willni does have both a mk5 GTI (an ED30 model) and a mk7.5 GTI Performance. However, I think their mk7.5 has the upgraded Dynaudio system, so any comparison they may be able to provide might not answer your query if the mk7.5 you’re getting has the standard system.
Don’t know if you’re part exchanging your mk5 against the mk7.5 but if you are, why don’t you listen to both systems back to back on the day you collect your mk7.5? That way, you’ll be in the best position to judge for yourself.