There are a couple of assumptions you've made which I'll address:
A 70-200 will always be more versatile than the DA 55-300.
I've tested both the Sigma and Tamron versions of the 70-200 against my own DA 55-300, and both simply outperform the DA 55-300 in every aspect even with a 1.5x TC attached to each (comparing equivalent FLs). This, of course, is an unfair comparison as the 70-200 is stopped down a couple of stops to compare with the 55-300 wide open. Nevertheless, there is no doubt a 70-200 opens up a lot more opportunity to capture concerts, indoors or out. Saying the Sigma is soft wide open is less important for you shooting at f/4 and smaller, and saying that the Tamron's AF is slow is in relation to what lens?!? The Tamron focuses MUCH faster than the DA 55-300, and quieter too.
Now the 55-300 is still a very decent and sharp lens, even wide open, and could do remarkably well for a *well lit* outdoor concert. As soon as you have overcast or late afternoon lower lighting conditions, you'll find that shooting at 150-200mm and beyond will require a significant (perhaps even unacceptable) boost in ISO to get a sharp image handheld.
Having said all that I have a number of sharp 55-300 samples, but not in your desired setting, so will not be exemplary for your purposes. These are both at 300mm (the biggest test of sharpness for a telephoto zoom...)
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