Actually, the mirrors on the F4U-1A/C/D would be quite useful, as there's three altogether: One across the top and one on each side. The side mirrors especially would be handy for those blind spots in the bubble-top Hogs you just can't adjust the seat around.
I agree with setting it as an individual preference option. If you want it, use it. If not, turn it off.
Since detail isn't an issue, the lowest-detailed wireframes can be all the mirror displays regardless of range. It'd be there for a quick check without having to change views, so even just a plain ol' dot with a con on it could do the job without eating up the frame rate.
The same case can be done for terrain: Minimal details only. Low-res tiles, minimal detailed wire frames, or perhaps don't show things like trees, buildings, GVs (if you're in an plane), etc in the mirrors at all.
This combined with scalable view ranges or details could help ease the performance burden.