....I don't think that there's any doubt that the GT Sports isn't a great Golf, but whereas it's far better than any diesel Golf, it doesn't compare to the GTI.
If you were buying from new and your budget couldn't stretch to a GTI, then that's a different matter. But you've experienced the GTI and unless you are simply never an enthusiastic driver, you'll seriously miss the GTI.
I know someone who swopped their Mk5 GTI for a GT and they are happy, but they are over 60yo and weren't comfortable with the GTI's performance or mpg. I'm over 60yo too but mentally I'm still in my 30's

. I don't mind growing old just as long as I don't have to grow up!
Btw, a Milltek exhaust will
NOT improve your mpg!! It's a
performance exhaust!!