These are the sample images from Tamron's site.
I thought this one looked very good for a 600mm zoom. Certainly much better than 400mm + TC.
Just brought that image (TIF) into Lightroom and looked at it @ 100%. It is stopped down a stop from wide open (f/9), but even so it looks pretty sharp. Individual hairs are resolved quite well. Even better, the transition zone looks good (and that was a real weakness for their 70-300 VC).