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You could use a classic up pipe as it has no cat. The size of the pipe depends on the turbo choice to be honest. Do the 04 WRX's use the TD04? if so stick with the same size as OE up pipe.
The magnex up pipe seems to be the one of choice. Has a similar diameter to the oem ones. I have a H&S up pipe on my classic and it is a little too big and spool isnt as good as it could be.