Hi,
you already can change from v40 to v35 using the FREETYPE_PROPERTIES environment variable. As for the Infinality parameters, I'm against adding them. They are misguided hacks that came to be because there is not a single toolkit in X11-land that ever rendered fonts correctly. They will actually distort the rendering and you still don't match the quality of font rendering of Windows and macOS. Qt 5.9 will ship with correct rendering for .otf fonts at least. Correct rendering is unfortunately outside the control of FreeType...