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The HKS Drager (not sure if it was a super drager, or if there's any difference anyway...) is/was a 3" the whole way through. The only non-standard part is the connection between the centre & backbox - the connection to the DP is standard 3" fit (not the wierdy Subaru 2.5"-with-o-ring sort of gastket, though). Just a flat fit. Same as H&S AFAIK .
H+S 2.5 inch or 3 inch downpipe will fit.
I have the 2.5. Sounds great - hear turbo spool up and car pulls so much better. Did not notice a big increase in noise overall.