There's already diminishing returns on Armour and Structure, so that's unnecessary. DPS limiting is also unnecessary, as once you get to a certain DPS the power soft cap becomes an issue, so there's no point. You never brought up output and block damage, but there's effectually a soft cap on that also (which is arguably more significant than DPS), due to a cap on how much damage one projectile can do, and how quickly the multiple output penalty stacks up. And shields don't really need a cap either, since capacity doesn't count for much really, and regen already gets hit by the power regen soft cap, whilst competing with other systems like Thrust.
So basically, no. None of these suggestions are necessary at all. Maybe a config setting to allow players or server admins to control how harshly diminishing returns hit.