I'm assuming you mean as his third wife. In which case Sardinia-Savoy would NOT be united with Austria. I assume this would be as a result of the War of Polish Succession, Charles Emmanuel aids Austria, maybe helps them hold onto Naples. In exchange he gets to marry Maria Theresa. The main problem is would he become Emperor after Charles VI? I suppose he stands as good a chance as Francis Stephen did. Long term I suppose it bodes well for the Austrian position in Italy. Though the variables involving Lorraine, Tuscany and how the Habsburg succession is actually handled could easily upset things.