It'll be a great deal more than 'just a loss' against Gryffindor and will have far reaching consequences.
I can think of a couple possible consequences, on one level, a drubbing like that -could- fuel the Gryffindor/Slytherin rivalry. But, OTOH, it could greatly provoke the ire of all of Slytherin House on the party responsible for so provoking the Gryffindors. The first makes for a very tense year, which would likely be rather a rather worse year than canon, while the second could make for some very interesting changes in what happenes; changes for the better.