I'm using a cheap H7 HID kit on my aftermarket headlamps, generally with HID's I would only tend to fit them if the headlamp had a projector lens (the round spherical glass ball which the C5 has), this focus's the light better and reduces dazzling compared to reflector only. Also you need a clear lens with no beam bending marks (usually associated with older glass lenses which the C5 doesn't have).
You also need to be careful when upgrading to HID's as there are legal requirements, firstly self leveling, my car is on coilovers and so my pitching is drastically reduced from stock and thus reduces the risk of dazzling other drivers upon front end lift due to acceleration, then there's the headlamp washer requirement, which I have not done (as yet). From what I've read like a lot of modifications providing you don't go overboard I'll run the risk, I've only gone for 6000k, OEM is ~4300k so slighly whiter, verging on a bit of blue. IMO iny more than this and it looks too blue and also looks stupid, not to mention asking to get pulled over for it.
If you are happy with halogen bulbs, stay with them, generally in the wet visibility is reduced anyway due to the light being reflected and refracted off the road and so looks darker, regardless of how bright the lights are to start with.
Just my 2p, I'll let you know how I get on, the chinese HID's aren't known for longevity (ballasts give out usually) but for £17.20 I thought I'd give them a try. Practically disposable at that price, if they last 2 years of so I'll be happy.