@KMKT Hi, and sorry for the late reply.
libarchive support is enabled in the official Qt IFW binaries, and for the upcoming 4.3 release it does the handling of all archive formats including 7zip. The reason it's behind a separate config feature is that libarchive requires linking against additional libraries in comparison to the LZMA SDK distributed with the Installer Framework, so we didn't want to force the changes to the users' build environments.
However, even after migrating the 7z format to libarchive, there have still been some findings about occasional memory allocation failures when extracting large archives. Moving the IFW binaries to 64-bit on Windows should hopefully finally fix these issues. The user story is tracked here QTIFW-2544, and is something targeted for Qt IFW 4.4.