Cold Cathode GU10 are available (small spot light) they are 230 volt I have not seen any 12 volt versions and would not expect to find any in the future the 230 volt versions normally ratted around 11 watt. LED versions ratted at about 3 watt are available in both 12 and 230 volt versions.
Some discharge bulbs can be dimmed but not all and where they use dimmers to dim the lamp the dimmer and lamp need to match as some need leading and others lagging types. There are types which don't need dimming switches it is all built into the lamp and flicking light switch so many times controls the level.
As to if they save energy it will depend where used, only if the heat produced by a tungsten is not required will changing for a discharge bulb save energy since discharge bulbs contain nasties and cost more to produce and can't be just put in the bin at the end of their life using them in winter months is not environmentally friendly. But they require less maintenance so I still use them means I pay more for gas central heating and less for electric lighting and gas costs less than electric per Joule used so yes they may save a very small amount of money but to save what the adverts claim you would need to use them where there is no thermostatic heat control.
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