Tamron's problem is that what made the 200-500 an option (cheap, light) might be lost with the addition of 50mm and VC. The 200-500 retailed at $1995 (though it obviously now sells for <$1000), and if this doesnt come in around that price point, what differentiates it from the various Sigma's, the 100-400L (and its possible update), etc?
I can get 150-500 and 120-400 for about $1000 from Sigma...I can get 50-500 for $1500. The Tamron is gonna have to be really sharp or really light