I just did this procedure on my '98 GT. It's an automatic and it didn't affect the shifting at all. I used the copper plug method and went with a hole a little larger that 9/32. I notice a definite improvement but I think I will try a little smaller hole at some point. The car is so much nicer to drive it's unbelievable. The car slows down just enough when you take your foot off the gas. I am not on the brakes nearly as much. Thanks to the guy who figured this out.