QTreeView very slow in OSX (known bug): possible solutions

Using a QTreeView on OSX leads to a very slow interface, especially if tree items are expanded and you perform scroll operations on the main window or the QTreeView widget itself. In Windows, everything runs just fine.
This problem is also affecting (always OSX) the QScrollArea and all the controls that have a scrollbar.
After some researches, it seems like this bug (going on from Qt 3.3), is caused by the QStyle::drawPrimitive().
Here is a thread at qt.nokia.com where some folks are discussing it: https://bugreports.qt.nokia.com//browse/QTBUG-9271

One of these guys suggests to modify the "PE_IndicatorBranch part of drawPrimitive". What file should be edited then? In the SDK I have an include and a lib folder, but no sources, so I believe I need to get those and search for the QStyle:.drawPrimitive, modify it, recompile and link the Qt libraries?
Or perhaps there is an easier way (overriding?) ?.
Thanks