Yes the Tamron 90mm is a very sharp lens, for macro shoots you will be using manual focus most of the time, especially if you are going to add extension tubes. So I think it's a suitable for TS's needs. Need not worry too much about autofocus. If there's any blurring, it's due to user problem
It's a great lens, but at 1:1, the lens body is stretched to the max. If what you want is 2:1, adding a raynox is possible, but the lens front element s too close to ur subject and if it's alive, you might scared it off.
if you just want a decent 2:1 macro, get a 28mm f2.8 manual lens with aperture ring(any brand, like under 100 sgd), a 49mm to nikon mount reverse ring, a manual extension tube, (under 50sgd for that accessory), you get 2:1 macro in decent form, check this
close-up filter decrease image quality a lot, if image quality is important, go with extension tube and Flash setup.
any lens can be a macro lens with extension tube. reverse lens way give you more working distance.