Beating LSU when we are double digit underdogs would put but at 20. The only thing that would hurt BYU's rankings would be to have more than 10 teams at 6-0 with all of them from P5 conferences.
But I'd this is not the case, by would easily make it to the top 10.
An underated season with that front loaded schedule would probably get them ranked 5th at season's end.