Sensor grade should also improve missile lock speed and turret tracking.
We once had a beta, where sensor grade actually did affect the angle gimbals were able to track targets in. When using e-graded sensors your weapons suffered a bit, d-graded already were mostly fine, anything above c-grade seemed to give you a little more of tracking arc than we had in the normal game.
Feedback from those who actually tested these changes were almost completely positive. Feedback from other people was very often negative. It was a small but nice advantage of higher grade sensors. They didn't give you a huge advantage, but at least there was a reason to have better sensors. I would very much welcome if this would be added again.
On the OP itself: limiting the range like that for mines would make absolute sense. Mines are cold and passive items in space, which indeed are hard to detect and the user would still need to be skilled to place them in the targets way. For missiles and torpedoes the proposed change is a bit more problematic. They are easier to use than mines and have high speed and significant range. Detecting an incoming missile (with active thrusters none the less) only at 250 meters out is too rough and only getting a warning for torpedoes, probably engineered to destroy the shield generator at these ranges is way too late.
In my eyes the suggestion should create an incentive to use higher grade sensors, but not make them mandatory. Limiting the range against missiles and torpedoes as described would require everybody to switch to a rated sensors and completely disregard any other sensor option. While the idea might also be applied to active munitions, the minimum range must be much higher than described. Even e-graded sensors should have enough range to give the pilot at least a limited time window to react on incoming missiles and torpedoes.
Without having calculated the flight times, by gut feeling i would say that e-graded sensors might detect missiles and torpedoes at like 1500 meters out, with every grade above that adding another 500 meters. Which indeed would mean that b-graded sensors (not that anybody would actually use those) would already give the regular 3000 meters and a-graded not giving a actual combat advantage in this aspect any more if you don't consider the mass advantage. (While a-graded vs. b-graded would indeed give an advantage against mines. )
TL/DR: I do like the idea, i just wouldn't want it to be as harsh as suggested to missiles and torpedoes, as i think it would shift balance too far in favour of torpedoes and would make a-graded sensors to be mandatory equipment.
