Sorry, just realised my post didn't make sense
but yes, the standard Subaru cat on the wrx and sti are the same unless the sti had the PPP sports cat fitted.
From that Scoobynet link - its definitely a sports cat so would be a good upgrade for you. The second pipe its bolted to with the heatshields looks like a standard centre-cat, but it could actually have a straight through pipe under the heatshields - also fitted as part of the PPP (which would effectively remove both the downpipe cat and centre cat from your car). Remap definitely needed.....
Re: uppipe, that WMS one looks pretty fat. The bigger the pipe, the slower the exhaust gases have to flow through it and the longer it takes to spool the turbo. I'd personally avoid and look for a genuine STi or Harvey uppipe.