secondary sites that list drops ( not sure how accurate the are) and on both BYU was listed as having in the mid to upper 20s for drops. One listed BYU as having 25 and another I think 28.
If you look at PFF grades for BYU WRs pertaining to specifically drops last season three WRs had grades in mid 50s which is below average to poor grades. One had a grade in low 60s which is barely average grade and two had grades in 80s which is good.
I think WRs had a lot of their drops with Retzlaff at QB last year because the ball was coming in hot. The game by game PFF grades show drop grades were lowest against WVU and ISU.