The actual effect and damage rate are not set in stone yet. Of course you are right, it won't be possible to create 90% damage with a set of cannons. Since we don't have the weapons programmed yet, we will program them and see how it works.

I didn't understand why you think a gravitron is an overpowered pickup... It only keeps your ship on the track for a few seconds like a mag-lock. Can come in handy if there is a tight corner with a hill in the middle so you can go around without slowing down but on the other side on a straight hill you will loose ability to barrel roll, so you have to make sure if you need it or not depending on the situation.
Phantomizer acts basically like a shield, but instead of shielding your real ship, it creates a hologram. So enemys will aim for that hologram since your ship is invincible. But still you can damage yourself if you crash into the wall. It's a replacement for the overused "god mode-alike" shield which you can find in almost every shooting game which has pickups so I also can't see how it is "ridiculously overpowered".

And for the Impulse. Imagine there are two enemies on your rear. Fire off an impulse will push them away. Same goes for the enemies directly infront of you. You basically made yourself room to fly and win some seconds of distance.