It's problematic because we don't know alot about Tobirama as yet, but thus far he is portrayed as incredible (able to break the Edo Tensei which Minato couldn't, until Oro got a power boost vis Hashi-cells). He also seems to think he can "take" Madara and Sasuke, and is generally hinted at being awesome. We need more feats, but I struggle to think Minato could do what Tobirama (plus 6 very powerful ninja) couldn't. Assumedly the 20 they were against were damn awesome (and no, Minato never fought the Gold and Silver brothers, that was years before Madara got the 9 Tails, as people have explained). I mean, how would Minato do against 20 people like Guy and Kakashi? He'd die.
Of course, this also presents problems, because if Tobirama could teleport, why didn't he take his squad back to base instead of dying? Why didn't he act as a decoy to hold them off, then after he had drawn them off, teleport back to base? Some hard limits need to get put on Tobirama's space time jutsu fast, or this will end up looking really silly.