Tamron 90mm. Teleconverter?

I've just pulled the trigger on a 90mm Tamron macro lens. I've read that a) its a good lens, and b) that focal length doubles well for both macro and portrait.
But for *real* macro work, I'm tempted by a 2x Tamron SP Teleconverter?
I've never used a teleconverter before.
I'm assume that you loose a few stops of light using one?
How 'small' am I going to be able to shoot in real terms? A fly in a frame kind of thing?

The Tamron 90mm lense is a 1;1 lense so yes you can even frame the head of a fly if you can get close enough. I use a 1.7 teleconverter for my macro shots because trying to shoot something like a fly's head, you're pretty much on top of it.

Should mention I have a TTL ring flash