Yeah it's a bummer that they don't dim but I have never thought I needed to. I have my brightness set at one click from the brightest setting for the OEM gauges. With that setting, the Prosport's match very well. During the day they are bright enough to see and at night they blend perfect with the dash still...
As far as the firewall, I still think that going through the drivers side wheel well is the easiest spot to go. The spot I used is easy to get to but almost impossible to get through. About a half inch behind that grommet I used is the heater box, so if you are to go through you have to make a sharp right turn down towards the gas pedal. The other kicker is that in between the back side of the grommet and the heater box is sound deadening material.
If you want to try that way it could be easy if you get a coat hanger through easily. After 10 minutes of struggling I would take the wheel well off and go through the grommet under there. That way you will have a straight shot to the under dash and fuse box for wiring.
With just the boost gauge, yes you will only have one small wire to go through the firewall. The rest of the wiring to the fuse .box will come from the back of the gauge
Here is another view of the hole before I went through with the group of wires.