See http://qt.gitorious.org/+qt-developers/qt/staging/commit/4a665ff5da05860f5eb46e3982ef3d8163a6cf59
The fact that QML doesn't support overloaded slots is not documented (and it's not only relevant for supporting "legacy C++ classes"; what about custom objects that are exported via setContextProperty()?).
QML will silently call one of the overloads (the first one in the meta-object); it would make more sense to throw an error.
Even better would be if it were possible to address the performance issues in QtScript, so that the logic could be shared.
Likewise QML does not support C++ methods with default parameters are these are exposed by Qt as overloaded methods.