I'm the OP and thought I'd give an update. I've had hard water in the automatic HP for about two months now. The only negative, if it's a negative, is the hard water overspray making the walls and surrounding blockwork white with the mineral and such. However, one thing that I've noticed as well, and this may because it's only been two months, I don't notice the FRP walls in the bays getting as dirty looking/that dark-ish water slime scum that builds up over time on the white FRP panels. For 30 years the automatic walls have required us to clean them periodically of this slime/dirt/crud from the FRP panels to whiten them up. The white FRP panels still look great after two months of hard water and not slimy. My guess is there's hard water building up on the FRP keeping things white. I'll clean the panels with my normal cleaners in a few weeks and see how they clean up. Still like the way cars bead-up with the hard water and how the glass ends up.
No negative consequences from the equipment, solenoids, swivels, etc.
Just my observations so far.