Well it usually advises not to because in some cases the extra intensity can damage cheaper projector headlight units and some say it because technically HID upgrade kits are illegal. I fitted a bi-xenon HID kit to my standard lights on my mk1 focus and have had great results. But if you want to stay legal I would go for some better bulbs. Philips Xtreme are about the best on the market with about 90% increase in brightness over standard bulbs. Using projectors will improve this further.