A friend recently introduced me to a PBS site feature "Do You Speak American"; in particular their "beastly mispronunciations" page. Imagine my delight when I spotted "Porsche" right between "Pecan" and "President".
In my experience, how you pronounce this word/name depends largely on whether you own the automobile in question. Porsche does not appear in any of my references, so I must rely solely on the evidence of my ears, which tells me that those who own a Porsche (or wish they did) tend to prefer the disyllabic POR-shuh, while those who don’t (and could not care less) tend to prefer the monosyllabic PORSH. Because the great majority of us don’t own (or aspire to own) a Porsche, I recommend the monosyllabic pronunciation as less ostentatious.