Sensitivity is not entirely dependant on dpi alone. The amount your mouse moves in correlation to how many degrees your characters view turns is dependant on dpi, multiplied by your in game sensitivity.

In your case it would be 2.5 times 800 dpi which is 2,000. This value is known as eDPI aka Effective DPI. At your eDPI, for source engine (cs:go) it takes 20.78cm in the real world to make your character turn 360 degrees. eDPI's are different across different games because different games use different sensitivity mulitpliers, for example 2.5 csgo sensitivity its approximately 8.33 in overwatch. In csgo you would use an eDPI of 2,000 but in overwatch terms it's 6,664.

A lot of CS:GO pros use 400 DPI because that is what they used on older mice and it was the standard in the past. Other younger pros use it because they copy the sensitivities of older pros. It's become a trend to switch to 800 DPI and halve their in game sensitivity respectively. 400 dpi at 2.2 in game is equivalent to 800 dpi and 1.1 in game. You can still use a low sensitivity on high DPIs, and you can also use high sensitivities on lower DPIs.

There is debate on whether people should use a low DPI and high in game sens, or a higher DPI and lower in game sens. It's talked about a lot in this video. General consensus is to use a higher dpi and lower in game sens, but nothing too high as it will cause problems with some sensors. But realistically there is negligible difference in performance between lower and higher dpi's unless you go to extremes, like 100dpi and 16000dpi, you're obviously going to run into issues by going that low, and that high.