I agree with VonMessa 100%
Compared to the things the average new pilot to AH has to adjust to. No tracers takes moments. Its part of what I teach every new pilot when we are up in the Val. Forget about looking for my tracers, you won't see them. Concentrate instead on watching me. If you see I'm going to line up, kick rudder, change pitch to wiggle and spoil my shot.
Forget about trying to learn to shoot by watching your tracers. Get behind the enemy, put the cross hairs on him, and pull lead in the direction he's moving.
Continue to hold down the trigger and pull lead until you see hit flashes.
Tracers just obstruct the view, and distract the eye from what it should be doing. (I think the same about most gunsights)
Teach your eye and brain to calculate, to learn what the right lead is for a given situation. Don't aim at the whole plane, (like aiming at a flock of ducks) Aim at 1 point and HIT that point till it breaks.
The secret to quick kills in AH is to keep your fire concentrated in the same place. Not scatter it all over from left wing to right, or down the fuse from front to back. Put your fire in one place, and HOLD it there till something breaks or he goes boom.
As for realism, crew chiefs made up ammo for their pilots as part of the normal process. They had many more options as to how they wanted it setup than we do. We have a simple on or off. They could not put any tracers in till say the last 50 rounds. (warning, about out of ammo!)
They could chose 1 tracer round in 5, 10, 15 whatever it was they wanted.
They could have tracers on wing guns but not on nose guns.
I would like to see MORE tracer options in game, not less.
As to seeing bullets, a lot depends on how good the light is, what your position is, and how good your eyes are. I used to be able to see the bullets from my .22 rifle going down range, could tell if I hit my target or not.
Now that I'm 55 thats long since a thing of the past.
For 7.62 (.30 cal ) MG it would be very tough. The bullets are small, and VERY fast. 12mm (.50 cal) it gets a little easier, they are however still small and fast.
For 20mm and up it gets a lot easier. I think some cannon rounds actually had a burning fuse in them (self destruct in case it missed? Or timed fuze?) Which would be quite visible.
30mm mine shells are a LOT bigger, and moving a LOT slower, yes you'd probably see those on a good day (with good eyes)