I got tired of suppliers having no idea about Variacs and their ability to dim various the types of lighting still available here, so I did my own testing.
The Variac I used is a 240-270v 2 amp "Dimmer Dot" variable autotransformer, which is actually an Indian product rebadged and sold by Australian Rectifiers (http://www.australianrectifiers.com.au). It was $120 plus shipping which seems like a pretty good price. It's big and heavy though, slightly too deep to fit into a 90mm wall cavity. Electrical noise was tested very unscientifically using a beat-up old Yamaha Pacifica single coil electric guitar. I tested the 3 main types of GU10 globes which are commonly available in our local generic-home-depot-hardware-house-store.
Results:
Standard 40 watt halogen GU10 - about $3 each
Dims evenly throughout the entire range of the Variac, from slight glow at 0% on the variac to full light at 100% No acoustic or electrical noise detected. Nice warm natural light. Runs very hot. I have some of these in the house on regular dimmers (hey it was like that when I moved in!) and they have an audible buzz when not at 100%.
"Dimmable" 11 watt Compact Fluorescent Lamp - $30 each
At 0% on the Variac, shows about 50% of it's maximum brightness. At 50% on the variac, reached 100% of it's brighness. Turning the variac from 50% to 100% made no noticeable difference. No acoustic or electrical noise detected. Virtually no heat. Even though it was a "warm white" globe, the light is fairly cold & grey. At maximum brightness, it was probably only about 75% as bright as the halogen.
When turned on with the variac below about 25%, it flickers like an old-style fluorescent tube. Certainly not dimmable as advertised, with a variac. Perhaps they're designed to work with triac dimmers, but as we all know that would generate all kinds of electrical noise.
2 watt LED GU10 - $30 each
This one contains about a dozen white LEDs. Interestingly, was the same brightness regardless of the variac setting. It gave off an awful, ugly light in the usual LED way, which started to drain my will to live. A bit less bright than the CFL.
Hopes this helps someone, information is very hard to come by on this topic. I'm still leaning towards the halogens and sticking with the variac, but it concerns me that if the government outlaws halogens, I'm stuffed.