I think it's just meant to be a given, unfortunately (b/c I really wanted this theory to work!

) Not only does James have the Gryffindor Quidditch trophy, but I remember when Sirius first described Hogwarts-aged Snape to Harry (I don't remember the text or have the books on me right now, though.) He said something to the effect of, "He [Snape] was part of a gang of Slytherins that all turned out to be Death Eaters." It kind of implies, the way Sirius mentioned
a gang
of Slytherins that he was talking about a House other than his own. If Sirius were Slytherin, too, wouldn't he have more likely said something like, "Snape was part of this group kids" or something more casual like that, because he would be referring to kids from his own House, that he knew personally? Does that make sense? He sort of implies that Slytherin is the
other.