If you can be bothered to get some VW door pocket adaptors, and run some speaker wire to the doors, you'll have a much better choice of speakers, and at 13cm they'll sound a lot better than those tiddlers in the dash!
Amps are not just to make speakers louder - most headunits don't have the power to make the most of quality speakers.
Having said that, the setup that's been recommended should sound just fine.