the BYU/UW game in Seattle back in 2008, I went with 3 USU grads (thanks to Boeing, there are tons of Aggies in Seattle). Two of them cheered their guts out for BYU and one of them was there to watch BYU lose.
One statistic that I'm positive is accurate is that there are more BYU fans among USU's student body than there are USU fans among BYU's student body. Of course, BYU has plenty of Longhorn/Trojan/Crimson Tide fans.