Ali,
I think the Tak 300mm f/4 is a very good lens, but it is not without issues. You will want to correct the CA with it near f/4 and, ergonomically, it is beautifully made but a little awkward to use. After removing CA, I'd say it legitimately matches just about any $XXX (vs. $XXXX) zoom at 300mm, and does so while letting about 1 stop more light through.
I'm a wide-angle guy so, besides the Tak, I only have a few other long lenses: Minolta AF 75-300mm, Soligor 400mm, Astranar 400mm, and Samyang 500mm mirror. Only the Astranar is bad, but none -- including the Tak -- can match the best of my sub-300mm lenses in IQ.
Some of that is probably because I hand-hold at shutter speeds well under the 1/f rule (which is hard even with sensor anti-shake) and some may be due to atmospheric haze. You might want to double-check that the Tamron is as bad as you think it is by checking it out on a solid tripod. However, your reason for looking at the Tak is why I bought mine....