Tammy 28-75 is sharp wide open, but it is indeed less sharp than itself @f/4 or F/3.2, perhaps you're expecting too much out of that lens. Maybe post some samples comparing F2.8 and f4 then we can judge the lens quality of your third copy
On a whim i bought the DA 40 ltd, and during my last vacation I ended up using it plenty. I actually didnt bring the Tammy because it's just so heavy (despite being the lightest 28-70/2.8 that there is), and for spontaneous street shooting it doesnt work for me. On regular use, the 28-75 did satiate my lust for the DA pancake trios for a while though; I keep telling myself "I'm not going to get any speed or range upgrade with the limiteds!". Compared to the DA 40, Image Quality wise the tammy loses at 2.8 and slightly loses in bokeh and color, but everything else about that lens is pretty much equal to the DA from my experience. It's a stellar lens.