Welcome nexus. I have done some aero mods to my Mk4 Golf. I did not really want to change the look of the car so I have done a full belly pan with an open section for the catalytic converter. The exterior of the car from VW is pretty good (except the back end).
https://ecomodder.com/forum/606678-post5.html
This shows a CFD of the Golf so unless you do some serious boattailing, the underside is the way to go, and perhaps rear wheel skirts. You can try smooth wheel covers but I have not noticed much difference with or without.
Your gasoline exhaust will be much hotter than the diesel so you will need to leave more open areas.