One of the most precise and best ways to transform geodata from the Austrian MGI-System to ETRS89 is using the AT_GIS_GRID (NTv2 Transformation) provided by the BEV. Thanks to the developers of the NTV2-PlugIn for QGIS it is really easy to use now.
Not everyone likes to manipulate the srs.db in QGIS with a SQLite-database editor to use the NTv2-Transformation with the AT_GIS_GRID like described here. The NTv2-Transformations PlugIn now supports transformations with the AT_GIS_GRID 🙂 – Thx to NaturalGIS
03.10.2015: Source-data has to be in epsg:4312 (may change in future).