Autobulbs site says this
As Dave C cays, bulb has to be like for like on original spec, also the headlight assembly should bear the correct E-mark, so retro-fit into a halogen projector, while it may produce a better beam would also fall outside reg's.
I've used HID in the Citroen-XM's I ran and they were a vast improvement, albeit not legal, but the original lights were that poor it was a necessary step. That car had self-levelling suspension though so no need for complex internals inside the lights
If you think the 156 is poor, trust me they aren't the worst out there!
On the current daily 156, with +90%'s in the lights are nice and bright, probably a lot to do with the outer film cover on the lenses being clear compared to its sister car (25K vs 134K miles but same age car). Both have higher output halogens in them and are workable for me.
As to whether you want to fit them in your own car, your choice really, this might be worth a read though...
Apart from the boy-racer brigade who tend to throw all sorts of junk on in the belief their underpowered cars will look kool and go faster, the worst for dazzling that I've noticed tend to be the high-end marques where they are
a standard and legal fitting