The problem with implementing this lies in the fact that as you said you don't think limiting turn speed is a good idea. This means twitch movements and quick sharp turns would have to be accounted for in whichever way this would be implemented, and most probably animated.
In GE007, it seems as if the camera follows the weapon's turning (instead of the other way around) as if the camera just lags a little behind the player model. If you set that lag, it doesn't matter how fast you're turning with the mouse; the lag stays the same and the turns speed isn't limited:
maximumCamAngleRelativeToGun == x; /* The faster the turn, the sooner the camera gets dragged to this max angle in time */
cameraCatchupDistancePerTime == y; /* For when the player turning doesn't cause the angle to equal maximumCamAngleRelativeToGun */
someCleverCode(x, y);
// Profit!
This is definitely something that could be looked at in the future though, I don't think it's a bad idea and it's certainly not impossible to implement.
Thanks for your considderation! :D
As for rail-shooter style aiming, that's a whole other kettle of fish. Probably 90% hinderence, 10% nostalgic.
Isn't hand-hold aiming supposed to be a trade-off? (limiting movement whilst increasing acuracy (for some))