Sure, you can use more points. The main reason I use few points is because we have to draw and manage every vertex by hand in Godot.
There is another option I didn't show you: if you select a sprite, a menu appears in the toolbar that allows you to generate a CollisionPolygon2D as a sibling. It will trace the sprite. But the resulting shape may not be round and smooth, causing some collision issues in game.
Oh and regarding performances, without measuring it, I couldn't say. The rule of thumb is to use the vertices you need, and to keep the collision shapes simple. We'd have to make a demo and measurements to see how costly they are.
But note you don't need a perfectly round collision shape for the slope to feel round for the player. In all games, the collision space is much simpler than what you see.