[quote=WinglessSTi]PDE makes a DP that does both but its expensive..the cat can be bolted on and off
We don't make that downpipe anymore - they're just too damned expensive to manufacture. Just sold the last one last week.
As far as HP numbers between catted and cat-less, it really depends on the cat being used. When it comes to aftermarket cats it seems you definitley get what you pay for in terms of flow numbers. Some aftermarket cats flow WAY better than others - the cat we use flows close to 600CFM @ 28" of H20 (when tested by itself), but they're not cheap for us to purchase - and that's reflected in the $559.00 price of the downpipe compared to $295.00 for the cat-less version.
As an indicator of potential HP loss from a catted downpipe, our flow numbers are as follows. These numbers were created on our Superflow 1020 flowbench.
Stock WRX Downpipe (Two Cats) ~ 260.4 CFM @ 28" of H2O
PDE Catted Downpipe (One 400cpsi Cat) ~ 414.1 CFM @ 28" of H2O
PDE Cat-less Downpipe ~ 475.8 CFM @ 28" of H2O
HP gains / losses between these different exhausts are directly relevant to the amount of boost / mods on the individual vehicle. An engine running 20psi will lose more running a cat than and engine running 16psi for obvious reasons. Surprisingly, with a good quality aftermarket cat, the WHP numbers between catted and cat-less are often minimal on the dyno unless some serious boost is being added.
I'd personally like to see everyone running a cat but we all have our own opinions and I respect that.
Hope this gives a little insight...