The majority of auto manufacturers use seam sealer for where panels come together and that may deteriorate over time. As stated above, I would start pulling trim and see if you have any water marks or rust to point you in the direction of any leaks.
My two cents. Save your money with the exhaust. Put aside those funds for an eventual down payment on a GT. Probably not what you want to hear, but this is experience talking
Just had a large branch fall on my roof during a storm. Roof is getting replaced. Yes, there is a CF option. Was looking at that CF option, still not sure.
Had a 2016 GTI with DSG for a daily driver and now a ‘21 6 speed GTfor a weekend ride. Both were/are lots of fun. Tough call between the two, but each serve(d) their purpose favorably