Question for discussion; BIAB to HERMS is an upgrade in what metric?
Don't know what you mean by "metric." I've done a lot of BIAB, and successfully; the one area where it's more difficult is in temperature of the mash. Not only is it more difficult to nail down effectively, you can't easily hold it. I lose maybe 2-3 degrees over the course of a mash, but if I want to hold a temp, well, that's tough.
If I end up too low, what do I do? I can turn on the burner, which may or may not get me there. And if I'm too hot, what do I do? Ice cubes?
There are several variables that influence BIAB mash temps. I brew in my garage and ambient temps can vary from in the 40s to the 80s. Further, I use a Blichmann Hellfire burner. That burner retains heat after I turn it off, significantly so. Thus I have to try to account for what I think of as "heating inertia" before settling at the final temperature.
Finally, if you want to do step mashes, doing it with BIAB w/o a HERMS setup is, shall we say, difficult.
Again, I've done a lot of successful beers with BIAB, but nailing the temp is not easy, at least in my context. Others may have better luck.
PS: I wish I had a HERMS. It would help with LODO processes.