Boise State is a good team and would do fine in the Pac-12. They would contend for the division and championship some years. But their record would probably not be as good as some of their supporters assume just extrapolating from their head-to-head OOC games with the Pac-12 because of the weekly wear-down discussed that they currently don't have to undergo.
I say probably only because I haven't seen any study on this, how playing tough teams every week affects teams physically and psychologically compared with teams that play in an easier conference (not that the MWC is much easier than the Pac-12 these days). The question is whether, when Pac-12 teams play Boise, they are in the midst of a grind and psychologically, it's just another game. Whereas Boise is fresher and circles the date of their match-up with the Pac-12 on their calendar. It seems like it's likely true but I don't know if there's any conclusive evidence for it.
Boise per Sagarin (which is finally getting to the stage in the season where it is worth anything in aggregate based on actual 2019 results) has the 111th toughest strength of schedule, whereas every Pac-12 team has a harder strength of schedule. But then... Boise State is undefeated.
Sagarin:
10. Oregon (SOS: 23rd)
18. Washington (SOS: 32nd) - I think perhaps a bit over-inflated due to their runaway margins against weaker teams.
24. Utah (SOS: 68th)
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25. Boise State (SOS: 111th)
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27. Arizona State (SOS: 44th)
28. USC (SOS: 12th)
35. Stanford (SOS: 2nd)
41. California (SOS: 14th)
48. Washington State (SOS: 98th)
54. Arizona (SOS: 54th)
70. Colorado (SOS: 36th)
75. Oregon State (SOS: 33rd)
80. UCLA (SOS: 5th)
Edited to add: as for Boise State joining the Pac-12, I don't know about that. Lots to consider. But the Pac-12 would probably have to move to 2 7-team divisions like the SEC, with another team besides Boise (San Diego State?) joining as well.