Don't know if you already decided on picking up the Tamron yet or not. I just sold one and purchased a DA 55-300mm. While I owned the Tamron, I enjoyed it and got some great shots with it. On contrasty zoom shots you will see purple fringing, but it is not too hard to fix. All in all I thought it was a great budget lens...... until I mounted the DA 55-300 to my K-x. In my opinion, there is quite a difference in overall image quality in favor of the DA 55-300. It looks sharper in the center and edges, colors are rendered very well, and the contrast looks great. Given the choice to do it all over again, I probably just would have bought the K-x with the DA L 55-300. I don't mind my DA L 18-55 kit lens and I think it is actually a pretty darn good performer also. The quick shifting focus and the original lens hood are nice touches but not a necessity. The DA L is optically the same as the DA model so you can get the same pictures. You could purchase a cheap lens hood from eBay for under $5 and 99% of the time I don't think you would miss the quick shift.
In my search to purchase a 55-300mm lens recently, I discovered a few companies advertising Pentax 55-300mm lenses (most of them say it is the DA model) on eBay. Do not be fooled by their low prices. They are all DA L models. One company (Ryther Camera from New Jersey) actually changed their ad to correctly advertise the right DA L lens. For $285 you get a new DA L 55-300mm with no box or warranty.
Adorama was running a special on the DA 55-300mm lens through Amazon.com for $339.90 + free shipping. For $55 difference, I sprung for the DA brand new with a box, lens hood, warranty, quick shift, and a reputable company to back me up if I have any problems. I would advise you to do the same if that is in your budget... otherwise I would try to get a used DA or DA L 55-300mm on pentaxforums marketplace. I have seen several DA L lenses go for $250 and DA lenses go for $280. PM me if you want to go that route. I know of a guy that is looking to sell a like new DA 55-300. I believe he wants $290 for it (shipped) but don't quote me on that. I could put you in touch with him and you could ask.
Cheers and good luck finding what you are looking for. The Tamron served me well and if you never check out a Pentax DA 55-300mm you will think the Tamron is just fine.
My lens collection started out with so many varied 3rd party lenses and slowly (and unintentionally) they are all being replaced by Pentax lenses. The last few that I have purchased (F 50mm 1.4, DA 50-200mm, DA 55-300mm) have shown me that Pentax is on top of their game in the lens department! For what I consider budget lenses, these crank out very high quality photos with my K-x like a copy machine... one right after another. Now I am so backed up with post-processing work because I am not used to having to process so many "keeper" photos after I go out shooting.
I will definitely be considering all Pentax lenses first from now on. It will save a lot of time, money, and shutter actuations in the long run.
-Jason