Charlotte hates them
me too :>
Luke’s suggestion sounds pretty good. But I would do it the other way 'round. Aim in the opposite direction you are moving. Press the mouse = fire + direction lock.
If you are at the screen boarders you need to be able to fire towards the center, that would work by either using a dead zone (a border you cant enter) or by working with deltas instead of absolute coords.
I would also turn the smoothing (speed cap) almost off. Heh it’s a mouse if you want it slow just move it slow 
This way it should be rather twitchy.
Oh and for the relative stuff… since you can be directly at the screen edges you need to mark the spots were enemys appear (small flashing arrow) or just blast em into the screen in places were you currently not are. But it’s nicer with random spawns imo, because it forces the player to move more - thus the perceived action is a bit stronger.
You could also use two different arrows. Simple ‘>’ alike thingies for slow appearing enemyies and bigger flashing (beep beep) warning arrows for enemy formations wich bust in with high speed.
Hmm… right now they spawn somewhere in the middle - effectively forcing you to the borders. That feels a bit restricting and is also kinda anti 360°.
The spread of the bubbles make it also hard to adjust the aiming direction. Those “bullets” are used as primary feedback for the direction you’re aiming at, therefore it should be a bit clearer (straight ahead).
Oh and if you want to know how good your controls are, make it one hit = dead. Then you’ll see. Btw having a energy bar in a shoot’em up, jump’n’shoot or jump’n’run is a two edged sword and in general it’s more on the bad side. In the design phase you won’t necessarly notice unfair things, suspense is softened, the amount of important decisions per minute is highly decreased and players with some experience will say things like “Oh an energy bar - i’m sure the controls suck.” the first time they see a screenshot. Well, most won’t say that but their intuition would tell 'em something like that.