Gruzze Tiakn - do leegst di nidaWhen you go back that far and to the various german tribes north of the Danube during roman times, then I don't think that Austria could really be considered part of Germany.
If you take the population into account that settled in today's Germany and Austria before the Roman conquest - the population culture would be quite similar. Basically Celtic groups - with an advanced culture and impressive fortifications/settlements. Mostly confined to today's Mid-Southern Germany and Austria. Due to the Roman conquest the today's Bavaria and Austrian people started to take shape. Whilst more tribes from Northern and Eastern Europe moved into the non occupied Germania. Therefore becoming less Celtic than those south of the Limes.
Maybe the last and biggest impact from 600AD onward was the migration of the Awaren into today's Bavaria and Austria. Followed by a 100 years of Magyar raids.
So the Tartaren (not Mongols) is often referred to by both Bavarian and Austrian history. (even resulting in a lovely food called Tartar).
Due to the following history of war's and invasions (most notable one being the 30 years war) there was automatically a large influx of "foreign elements" into the Bavarian and Austrian population - resulting into today's Bavarian's and Austrian's.
Furthermore the "gift" of Napoleon (Franken) enlarged Bavaria's territory and population by almost double - with those Holb-Preissen aka Germanic Franconians.
So Southern Bavarian's and Austrian's are most likely identical besides some minor dialect issues and habits - but both are related to these Germans