Constant Vsync on, QGLFormat::setSwapInterval() value ignored

Self compiled Qt libraries with -opengl desktop, release build. Rendering is done in a separate from GUI thread. Before any GLWidget is created following code is executed:

QGLFormat fmt;

fmt.setAlpha( true ); 
fmt.setDepth( true ); 
fmt.setDepthBufferSize( 32 );
fmt.setSwapInterval( 0 ); 

QGLFormat::setDefaultFormat( fmt );

Even if setSwapInterval is 0, rendering will still be limited to 60 fps. Setting 1 or 2 instead of 0 gives the same result - 60 fps.
Qt 4.8 had no such problem. Full rendering speed in Qt 4.8 is around 200 fps.
The following problem rises with NV driver Vsync option set to "Use 3d application setting". If in NV driver Vsync is set to "Always off" then rendering can achieve full speed.