Keyser I totally agree with you that kit is sharp enough at 8-11 however it lacks those extra 15-20mm that come handy in portrait. Besides fast and accurate AF is a must when shooting tods, bright lens is also useful when you decide to shoot with modeling light only and width is handy for small families.
I think that Tamron 28-75 is the best option for me and I saw many brilliant studio shots with it. However I am not too good buying and returning stuff and dealing with angry shop assistants if I get a lens that does not fullfill my expectations. Life is tough enough...