To put it simply, it was the best place for me. I tried A-Pillar, lower-left dash, and instrument cluster, but found that headliner/rearview mirror was best. I’m tall so my eyesight is up there anyway.
I have an A-pillar gauge holder if anyone wants it.
The end result is a nice-looking, nice-functioning boost gauge that dims with the rest of my 1997 850’s instrument gauges. Because it’s mounted up high it’s unobtrusive, and far less likely to be seen by street racers, cops or thieves. It’s there when I need to know boost, and invisible to me all other times. It’s slightly nearer to my field of vision than any of the other mounting positions I tried, so my eyes are on the road more.