volt gauge in the supra is worthless IMO altogether. The car does special things when voltage drops to warn you. (unless you get it for free)
Boost isn't really needed eighter. But "nice" to have. But also, proves itself to be worthless.
IMO, you should have a Oil Pressure and Water Temp. Fuel Pressure is nice too, but also proves itself to be worthless. Oil pressure is also worthless if your factory sender is new.
The factory water temp is crap on 95% of cars. As they don't want you to get scared as the temp changes.