The standard speakers aren't bad tbh. The standard headunit is a bit weak so you don't get the most out of them. Mine sounded much better with an aftermarket setup too
Wiring loom @ 13.99Ariel adapter @ 9.99Fascia @10.99
Also dont forget if you have an early model GTI MY05 you also get the bigger 8 (or 10? I cant remember now) speaker setup