The Torah in Parshas Pinchas tells us to bring the daily korban tamid "baboker," each morning, but with respect to the korban musaf of shabbos it uses the expression, "b'yom hashabbos," on shabbos day. Based on the difference in language, the Rama (O.C. 281) quotes from rishonim that our davening, which corresponds to korbanos, should be scheduled later on shabbos than during the week. (Even though the word "yom" is used with respect to the korban musaf, the assumption is that the musaf immediately followed the tamid, and so a later musaf implies a later tamid/shacharis.) The Mishna Berura notes that later davening does not mean so late that one misses the zman of kri'as shema, but simply later than the normal weekday tefilah.
Is it permissible to daven at a hashkama minyan on shabbos? It seems that this din is not a real derasha, but is simply an asmachta. Davening later is a kiyum of oneg shabbos -- most people are happy to have an extra hour or two of sleep. However, if davening earlier brings you more oneg shabbos than sleep, then I imagine (you can ask a rabbi to be sure) that there is no issue.