When going manually, follow the blue marked step while FSX is not running.
On the next start, he will recompile his shaders then. You will see a progress bar there, to be sure it happens.
As the description above is a bit cryptic, just use C:\Users\YOUR USERNAME\AppData\Local\Microsoft\FSX and look for all Shaders folders there. Delete or move them and then start FSX.

The installer should already have taken care of that, so maybe you actually did get new shaders, but it did not help much on your card.
I hope I'm wrong though.