The descent speed can indeed be increased. However, the X and C are okay, they are not supposed to have much effect when you have reasonable horizontal speed. This was the same in Arma 2 and TKOH, if you want to turn you should tilt the main rotor.

Why isn't rendering grass up to x meters a solution? My already quite old rig (Radeon HD4890) performs fine when rendering grass at the maximum settings in Arma III, so for me it's a solution. Up to 600 meters might be enough for most infantry combat in Arma III with no significant performance effects.

Furthermore, sinking or fading would then be dependent on the kind of terrain somebody is. Fading should only happen on areas with high grass and bushes, not on concrete, roads or low grass.

Instead of "setting to max" I would argue that having a minimum draw distance is much more useful. Drawing grass for somebody at 3 km is already irrelevant as he would be too small anyway from that distance. Perhaps drawing grass for 1500 m is enough, perhaps even 1 km or lower, although the most optimal distance can be finetuned.

I completely agree however that having no grass on distances lower than 500 meter is not advisable. Furthermore, providing sliders in the option menu is nice, but these options should be server sided. Clients shouldn't be able to adjust the grass settings themselves to make camouflage and going prone in high grass much more effective.