I call BS, im more than sure that AT ANY WAY to see behind you was a GREAT help no matter what.
The field of view from the mirrors was so small, coupled with the vibration of the aircraft, that the only thing that they were useful for was for telling you that it's time to strap on your 'chute because it's time to bail.
I recall a Luftwaffe ace, I think it was Rall or Galland, saying that by the time someone was close enough and in the right position to be visible in your mirror it was too late.
Although, I'm sure you're more familiar with the use of mirrors in combat than someone with 275 aerial victories, or someone with 104 aerial victories and who was head of the Jagdwaffe, respectively.
As far as frame rates go, according to HTC for them to work properly you'd have to render the entire environment twice, which would literally cut your framerates in half. I run Aces High fine at 60 fps, but 30 would really suck.