Being able to shoot faster by pressing the shoot button repeatedly was an intentional choice - you get a benefit if you put in the extra effort =)
You are not imagining things. Each saucer ship is giving a random speed from a limited range to make them more interesting. Also, they are now each a different color so you can better tell them apart. There is also a little arrow indicator that circles around your ship that points to the nearest enemy saucer ship (or the green asteroid) to help you track them.