Hey,

so I was wondering if anyone could tell me how much of an impact coins actually have. And is it just higher max speed or also overall higher acceleration with more coins?

Also, sometimes one does seem to get some kind of speedbost with some dashing special effect in front of the car. Something that doesn't have to do with an item or speedpad etc. When/How does one get that?

The boost you're randomly getting is likely from driving behind somebody long enough. You'll see the lines appear beside you, and if you trail them a bit longer you'll get a crazy huge speed boost. It's even dangerous if you're not prepared.

As far as coins go, they definitely make a difference. If you're ever sailing along with 10 coins and suddenly lose 5 to some unfortunate circumstances I bet you notice how much slower you go. I have no clue on the actual numbers, but it's noticeable.