Do not overestimate Tamron's weather sealing. My recent experience tells that both sealing against dust and humidity are close to none.
Would you care to elaborate? I have shot with the Tamron in pretty extreme conditions (from -40C to 40C) and in snow, rain, freezing rain, etc... without any kind of issue. I am interested to hear why you would make such a statement.
I used my Tamron for taking pictures of recent floods in Central Europe. It was quite bad weather (small to medium raining almost all the time). Even with umbrella my lens wasn't entirely protected but it wasn't exposed directly to the rain. It took just few minutes before the distance window on the lens get misted but otherwise the lens performed quite well. The problem appeared when I put my lens back to the bag and took it out later in the office. One of inner elements was completely misted as well. It took about two hours before the mist disappeared. The temperature difference between outside and office was 10C to 15C so I would not consider it as really high.
I took the lens to the service center and complained about the issue. They informed me that it is normal and it is called physics because any lens which extends during zooming must pull air inside and with air comes humidity. It was not considered as valid claim for warranty replacement / fix.
I have also find dust particle directly in the center behind the front element. Again it was considered as normal even I really argued about that with 5 month old lens which claims to be sealed.
I left the lens in service center anyway because AF motor started squeaking recently. My case may be special because this is the third time my lens went to service center for warranty repair and they at least once opened the lens (so I believe they are directly responsible at least for the dust).