flat slides in a round slide... flat vs round

Can't answer those questions. I would think (not sure why, though) that the cutout actually is less turbulent for the air than the flat edge, but I have nothing to base it on.

I think we'll need someone more versed in these topics to chime in on what the functional difference between cutout/no-cutout is. Larry?

the benefit of a flat slide carb, is its ability to draw fuel from the jets easier at lower vacuum (low throttle openings).
As the carbs diameter get bigger (by putting a larger carb on the same bike) the vacuum signal gets weaker at low throttle openings. So a carb with a flat slide will give u better throttle response at low throttle than a round slide of the same size (same diameter opening).
A small diam carb performs better at low throttle openings and a large diam carb performs better at large throttle openings.
So the flat slide helps u use a larger carb .