Hey! Thanks for playing and providing your feedback. I agree the sprite should change when powering up. This has been fixed in the latest update so you should now see a visual change on each power increase.
Regarding shot speed, this is actually technically already in the game -- the closer you are to the top of the screen, the faster you shoot, so as to reward riskier playing. The effect may be a bit too subtle though.
I'll definitely consider this for future versions!