Here some tweaks to improve framerate.

[Rocka]

I had problems at first getting it to work properly even though I did everything according to the original post. Didn't notice any improvement at first try.

Then I read somewhere that the cache file in the game folder could be the problem.

So what I did was:

In steam - Right click on the game name under your Library, then choose -> Properties -> Local Files -> Defragment Cache Files.

Went from ~25fps to 65fps average in soloflight, pretty impressive boost I'd say.