Utah has won against BYU every single time since joining the PAC12. So that's one conference loss for BYU and one conference win for Utah, each time they play.
Do you really think that a Jake Heaps/Riley Nelson BYU team would have done better in the PAC than Utah? Do you think Taysom would have stayed healthy in the PAC? Do you think BYU's mediocre defense (picked apart by MWC and C-USA teams last year) would have held up against PAC12 schools?
Utah hasn't exactly set the world on fire since joining the PAC12, but let's not be delusional - BYU would NOT have done better.