BOOM!!!
That will be what happens eventually if you try amping the VW speakers with any kind of decent power with a signal from an aftermarket HU. 
Which is what Im doing now. I think I better add this to the list of "presents no one will buy me because they are too expensive so Im going to buy it for myself this xmas" list 
.
lol.
The stock speakers will hold up for a short while at least and so you could always test them to destruction
You will instantly notice that they are louder and crisper, but you will also notice quite a bit of "noise" at certain frequencies.
These frequencies are metered out with the stock VW head units as they employ software (DSP) to process out the frequencies that damage the speakers over the long term. Adding an aftermarket HU and then pushing extra power through them with an amp is going to end up damaging the VW speakers. They may not actually go BOOM, but the end result would be the same.
