I tried doing click to shoot at first but my finger got tired, so I decided to go with automatic. I'm not sure what the aim of speeding up the projectiles would be if the enemies would be empowered to match. Do you feel that the game pace is too slow?
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You could make it a press/hold thing as well. The speed up thing is less of an issue for me, it's that you don't have control over when you're firing, which makes aiming the projectiles more difficult.
Speeding up would also make it easier to aim since you wouldn't have to lead the shots as much, but you'd also have to adjust the difficulty a bit to compensate. I prefer faster feeling games myself so that may be my personal bias talking :P