well adding a set of MSDS headers can net as much as 15 horsepower, so I"d say that's a pretty strong choke. adding SVT (bigger) UIM/LIM compenents is also pretty noticeable, since the sizes are pretty different. A lot of the other stuff you listed is negligable, but a modified Y-pipe is also a highly suggested mod, but it doesn't surpass the headers.
and fyi, you double posted this thread
