In the post Lavell era BYU is 10-3 against P5s in the season opener. There's something about BYU coming out hard in the season openers. That said, recent history against Utah has the opposite story. Hopefully, the season opener trend holds while the Utah one doesn't.
2018 vs AZ - W
2016 vs AZ - W
2015 vs Nebraska - W
2013 vs UVA - L (Weird game, lightning storm)
2012 vs WSU - W
2011 vs Ole Miss - W (Thank you KVN!)
2010 vs UW - W
2009 vs OU - W (Max Hall: We will win!)
2007 vs AZ - W
2006 vs AZ - L
2005 vs BC - L
2004 vs ND - W
2003 vs GT - W