The problem with HIDs that require ballasts (i.e. true HIDs), is that if you install them WITHOUT the corresponding projector lamp system, you do end up with a brighter light, but also light that does not have the correct cutoff or focus and thus glares other road users.
This is because the standard "normal" headlight reflector is designed to scatter the light in a certain way, whereas HID's require focus which the additional projector lens supplies. You can spot a projector lens as they look like one of these sitting inside the headlight assembly:
I'm pretty sure that not having the projector system to "control" the HID light is actually illegal in the UK and therefore, if stopped, and PC Plod notices you have the ballasts but no projectors, you'll be in trouble.
The ONLY way to install HID's correctly is HID's + Ballasts + Projectors ....i.e. a proper
retrofit which would involve ovens at 230degC, softening sealant glues, cutting/dremmelling/etc/etc - time consuming and messy.
Alternatively, you could buy "pre-made" projector lamp assemblies for the Jazz on Fleabay but they're VERY expensive and usually look uglier than the stock headlamps.
HID's + ballasts installed into stock headlight housing "as is":HID's + ballasts installed in stock headlight housings that have been retrofitted with projectors:So without a proper retrofit, HID's are actually blinding and therefore not useful anyway (more likely to cause an accident than see one coming).....
.....and if you're not prepared to take the whole headlight apart and get jiggy with a hacksaw, I'd second the Philips Xtreme bulbs myself - which are direct drop-ins
designed to work in OEM headlights without mods and even though not as bright as properly installed HID's, they're at least noticeably brighter than stock bulbs
.