Well if your staying OEM you probably have the best head unit. Spend a bit (500 or so) and you could have a great double din from alpine, clarion, pioneer etc.
Speakers are a big let down on the mk5. Although all cars have grilles for 3 speakers in each door, most only have the tweeter and mid bass units fitted.
Going for 3 way replacements would be the first move with a mid in the factory location and some well placed sound damping measures inside the door card.
Hertz, Focal and JL do kits that fit.
Can't comment on the first 2 but I have the JL C5 653 set and they sound great.