Eric is right... If the correct retrofit process is per oem spec or even exceed oem
then there is no difference between the oem and the retrofit.
Of course, if you reftrofit it improperly like: fitting rebased HID D2S bulb kit into current halogen enclosure, this is illegal!
Cops, will not caught you, as they most likely will not have a clueh what is the 'right ' HID and wrong 'hid'. But if you are involve in an accident and this improper retrofit of HID is the one causing the accident (too much glare, blinding etc), then you can be sued.
So, if one does retrofit HID, be carefull ~ When i say proper, it means:
- use hi quality bi-xenon projectors in your halogen headlamp. These are the cause for the bluish difrraction of HID beam. Not the blue bulb those honda rice boy put into their heavily mod civic!
- properly aim your HID with proper cut off ________/-------------
- elimnate the reflector surface in your current halogen headlamp.
hence mute the 'glare-problem'
- must not loose your hi -beam (this mute the fitting of rebased HID kit, because if you use 9007 HID kit, you will loose hi beam).
NO DUAL FILAMENT HID BEAM EXIST OUT THERE.
UNlike 9007 halogen bulb, it is dual filament right?
- buy hi-quality ballast like philips or hella so that it wil shutt off by itself during accident/impact, hence no electric shock potential.
Philips and hella ballast are used in most oem xenon: audi, bmw, mercedez
- safely mount the ballast per oem spec
- put relays and proper wiring harness between power source VDC and the ballast
Sound like oem ?

oh yeah... The cost: i did it on my audi, cost around $1,100. Hope this eliminate the hesistation to retrofit!