Two points worth noting;
a) In some countries, it is illegal to use bulbs that are not approved for the lamp/headlight unit.
b) Headlights, both reflector and projector types are designed to emit light based on the light source being at a known point inside the lamp. Bulb specs (such as H7 and HB3 the lights used in the 166) specify exactly where the filament is located in the bulb, so the light within the housing can be focused. That’s partly why you see after market xenons look like they are emiting a huge ball of light, these xeonons have a light source location that is different to the halogen bulbs.
That said, the standard halogens in the original 166 are very dim. I’ve tried xenons in the standard halogen headlights and they throw a huge amount of bright blue light out, great for driving on dark country roads, or out of city motorways but dazzle oncoming drivers and are illegal now here in Ireland.
Lately I’ve switched to LED’s, going for branded ones that have the light source in the same location as the filament of a halogen, the Philips Lumiled range. Not quite as bright as they xenons, but much brighter than both the standard halogens and the modern brighter halogen bubs such as Osram Night Breakers, but also not illegal here.
My advice, don’t don’t go with after market xenons, consider either modern extra/ultra bright halogen bulb options or branded LED’s that conform to the standard for the light source location.