my advice about dimmable downlighters is not to have them on the ceiling beside the attic.
It means that the ceiling will not be airtight (heat loss, also condensation will happen in the cold attic). You'd never see dimmable downlighters in a passivehaus.
Also regarding "dimmable", only incandescent bulbs and halogen are dimmable.
Both these types are not energy efficient........they burn 4 or 5 times more current for the same light as cfl or Led.
If you want to lower your energy use, forget about dimmable. Leds are not dimmable. (Some very expensive cfls are dimmable, but they are not widely available)/
Id say chose lighting which use cheap and widely available CFLs (Edison screw, Small Edison screw, or bayonet. This is exactly what I did. My lighting bills are very low, replacement bulbs are cheap, and my ceilings are airtight.