Consoles add their effect to a lot of other effects that also give +damage%, notably your Starship Weapons Training and Starship Energy Weapons skils. That's usually quite a lot already, so adding 30% to 300% already in effect, you'll see only a net increase of 10% ((330 / 300) - 1).
Adding more consoles on top will add the same increase in absolute damage each time, but that increase will make up a smaller percentage of your already existing damage bonus.
Hmm, that makes sense, thanks all! Guess I wonder what is better, using these +damage increase consoles, or consoles that increase weapon power ...
Since +damage consoles are Tactical, and +weapon power consoles are engineering, you could actually use both.
For Tac, there's really nothing better to put in there - just stick to one energy type, stack consoles for that type of damage, and you're fine. No way to improve upon that, really.
For Eng, you'll have to balance that against your need for defensive consoles or special consoles, so that's a bit more difficult. There's a hard cap for weapon power (and all other system power levels) at 125 though, any surplus is wasted unless it is a temporary increase (from batteries, EPtW, Eng's EPS Power Transfer, drain ...) - on the other hand though, you'll always want to get as close to that cap as possible without causing spill-over. For most ships there's better options to do so though, so you won't be using weapon power consoles much, if at all - though they are helpful at lower levels, and some builds will get stuck with having one of them even at VA.