Before you try and crucify me, allow me to elaborate a little.
Rapid Fire may not be overpowered across the board, but it's too much burst damage in its current implementation. To me, too much burst damage is exactly what ruined Arena PvP in World of ******** - going from 100% to 0% in ~5 seconds was dumb, and happened even in top notch gear. I've seen the same thing happen sometimes in STO, and it seems to be almost exclusively due to Rapid Fire.
Now, a straight nerf to rapid fire would probably be a bad idea, and may encourage people to ditch cannons all together. I don't have any specific numbers to suggest, but perhaps reducing the rate of fire and increasing duration would be a decent solution. Or maybe add some secondary effect while reducing the damage it deals. Either way, the front loaded damage is simply too high, and will really start having an impact in future organized-vs-organized games (specifically by giving the Klingons too much of an advantage).
Extend Shields gives too much regeneration/healing with no noticeable downsides. In fact, it's not so much an extension of one person's shields to another, as it is a straight shield heal. I can go from, "Oh no I'm about to die because everything is on cooldown and I'm getting hammered," to, "I can go afk and still live." (that said, I still <3 people that use Extend Shields on me)
I think this ability needs a change so that it actually is an extension of shields, at least to some degree. Say...gives some shield regeneration/healing (less than current), and also redirects X% of damage back to the shields of the person doing the extending. In this way it would still be quite helpful as team support, but it wouldn't be near invincibility, and the user would have to be wary of getting switched to.