Here's what Mizzou's scoring D has done (compared to the opponent's scoring average)--note that I left the FCS team out (because FCS).
Ark St. scored 20 pts, (16 below average)
UCONN scored 9 pts (10 below average)
Kentucky scored 21 pts (right at its average)
South Carolina scored 10 pts (11 pts below its average)
Florida scored 21 pts (7 points below its average)
Georgia scored 9 pts (20 pts below its average)
Vandy scored 10 pts (6 pts below its average)
Miss St scored 31 pts (3 pts below its average)
Quick and dirty math shows us that, on average, they hold team to 9.125 pts below their scoring average for the year, with a best result of 20 below and worst of no difference. So if we project those numbers onto what we might expect out of BYU (who averages 36 points a game), if they hit their average, BYU scores 27 points. If they match their best result, BYU gets to 16, if they tie their worst showing against scoring averages, BYU puts up 36.
On the flip side, Mizzou has played 4 scoring defenses ranked as high or higher than BYU: UCONN, Miss St, Vandy, and Georgia, scoring 9, 6, 3, and 13 points.
Mizzou will have to play out of their minds on both sides of the ball to make it close, much less beat BYU.