I wasn't really advocating wearing jeans and a T-shirt in SE Asia. I'm just saying that, in my humble opinion, people who wear those long sleeve shirts with the pockets and the convertible pants with those big hats look ridiculous. I mean how often do you really unzip the pants to make them into shorts?

I think you would be fine bringing a couple of thin short sleeve button down shirts, some shorts, and thin pair of pants. You don't need to spend $100 for an ex-officio shirt that screams tourist.