Its a bit better resolution and contrast at the long end than the standard, but it also seems to vignette a bit more wide open (not something I care about much, but some do). I've never shot the Tamron, so no experience there at all.
I don't want to form an opinion on just one copy: this one may not be up to snuff and the ones coming next week better. But on 3 different cameras I'm just not $1,500 impressed yet. Maybe this copy just isn't what it should be.