Everything you mention was for a true freshman straight off the mission. I believe there would have been improvement given he is removing his mission legs.
Secondarily, he would have been getting better for next year. He is not going to progress riding the pine as much as he would if he played. He is going to have some rust in 2017, he would have been better had he played all this year.
IMHO, BYU would have at least two more W's had Tanner been the QB. Having a quick release, knowing where WR's will be would help immensely.
Taysom is an animal, great guy and person, he is just not out best option. I have zero against him, his wife's family are friends, I believe Tanner gives us the best shot to win short and long term.