What is the consequence to switch OFF the DirectX 10 for another add-on?
I don't know of any addons that requires DX10, many (especially airports) have issues with it, and the one compatible with it looks the same as in DX9.
I don't know when the DirectX 10 is OFF, if this will decrease any video performances or any special feature options on several sceneries that I have installed.
The only thing that badly affects performances in DX9, is Light Bloom. It's much faster in DX10.
So, if you really can't live without Bloom effects, and want to fly over Geneva (and many 3rd party airports that doesn't work in DX10), you'll have a performance hit in DX9 if you turn it on.