germ, there's generally a big difference between stock and aftermarket HIDs. Color temperature is certainly one aspect. OEMs typically select non-blinding whiter light, rather than the bluish hues seen from both aftermarket kits and even aftermarket halogen bulbs (HID lookalikes).
Another factor is that the lights are incredibly bright. Aiming of the lens is very important. Even just a little bit off and oncoming drivers are cussing you up and down. Most OEMs include small leveling motors and even ones to slightly aim the bulb left or right during turns. The VW GTI is one such car. If fat people get in the back, then the bulbs aim a bit more down and keep them out of oncoming eyes. Aftermarket kits don't typically do this. Some do, but at that price point, it's easier to find an importer with a source at an Asian VW dealership.
I spent 8 years in Taiwan and Korea and know that small cars rule there. They have money to buy nice ones, but no room to park big ones. Small cars like Golf and Jetta come with LED taillights, HID headlamps, REAL leather, navigation, etc... standard!! If you can find a source, you can nicely upgrade your car. It won't be cheap, but it will be with VW parts and will be right.