Ext.Panel does not have a title property so calling setTitle will not do the trick.
But the tab title is given by:
on the tab panel add nethod.
tabTitle = (card.getTitle) ? card.getTitle() : initialConfig.title,
So if one would extend Panel in such a way that it will have a title the upper code might have sense.
Also another cute thing about cards added to tab panel is that they receive a reference to their tab:
So a solution
card.tab = tabInstance;
Now this should help also if you want to change panel title ~ tab title at a later time, helps a lot when you have an application localized
if(newTitle && this.tab)
this.tab.setTitle( newTitle );