I'm in the "Porsch-ah " group. I've found that, if a stranger approaches you with questions about your "porsh" and you are friendly and enthusiastic, but subtlely corrective, they will change the way they say it... at least for that conversation.
Example: (Random encounter late this summer, filling up at a gas station near my office; heavily paraphrased)
Me:
Guy: "excuse me - is that your Porsch? is it fast? How much $$$ is a Porsh"
Me: standard answers... working in the correct pronunciation; something like "...Actually, you can pick up a Porsch-ah used for a whole lot less than you might think..."
Guy: "cool, I'll check out porsch-ah's on Ebay"
I think it's kind of amusing how many people will adjust their pronunciation to match mine; I've almost made it a game.
Then again, I've also used "Porsh" at PCA events just to see what sort of reactions I get, so maybe I'm just a troublemaker... 