I'm curious here...I can see the point everyone's making in regards to leaving the headers as is to form a seal that's at each opening versus forcing a seal across the entire surface, but when I look at a lot of other headers I see a smooth surface all the way across. Below is the shot of Porsche 911 stock headers on the left and aftermarket ones on the right. Stock has perfectly smooth mating surface and I don't hear of owners complaining about leaks. The aftermarket ones have individual mating surfaces. Maybe that's what needs to be done for the ISF?
And let's just assume that yes, it's better to have non-smooth ones, the openings to the tubing seems pretty crappy anyhow. Shouldn't they still be nicely rounded out regardless of the mating surface?
Those aftermarket Porsche headers look sweet. The holes look about a quarter to third surface area larger than stock.