Its all personal choice, but just to throw my personal experience into the pot...
I've has a 2016 GTI on a lease for the last 2 years. I went for just basic spec (as it was a lease) and didn't regret it at all.
I went from an Audi A5 coupe with 19" wheels and 18 speaker B&O sound system to a standard golf and was quite happy with the sound system. (If you want to throw £850 at the extra sound system, I'm sure its great - but I went from the premium Audi system to the standard golf system and I've never regretted it. - But then I'm 58 and don't play it with the bass racked up to 100db)
Also, I went for standard 18" wheels and no dynamic chassis - and never regretted that either. Much more compliant than my Audi. Again - we're all different, but even over crashy local potholed roads, I find the standard suspension, on 18" wheels is fine for me (and for my wife...)...
I was so happy with those choices that I didn't change them when I ordered my new 2019 GTI PP.
The one option I did gamble on for my new GTI PP was the Art Velours interior.
The tartan interior is fine but it was the only thing that I used to look at in my old GTI and think could be improved.
I didn't want full leather - both for the cost, but also because my wife's biggest grumble about my A5 was that when I was driving it "spiritedly" she kept sliding from side to side in the leather passenger seat.
She was so much happier with the cloth seats in the GTI because she wasn't sliding from side to side as I was cornering...
The choice was a gamble because the first time I saw the Art Velours interior was when I collected my new 2019 car - but I think its fantastic.
The "leather" inserts in the doors and on the door arm rests, with contrasting red stitching takes the interior up a level from my previous cloth GTI; and the "pseudo-alcantara" seat faces are just lovely - both to look at and to touch.
After a fortnight with my new GTI PP, I've got no "spec regrets" - but, as everyone says - we're all different, with different budgets and there are no right or wrong answers...