Thank you schoellnasty and Matt for posting that so I don't have to rip into a n00b

Warning: content ahead

On a turbo car, you ideally want ZERO back pressure after the turbo. The turbo itself works on the pressure differential between the inlet the outlet to operate. The turbo's resistance in the exhaust stream will provide the back pressure that the engine needs. Running an open downpipe will not increase your overall boost, but it will make the car slightly faster because it will spool up quicker. Of course, this means nothing if your ECU tune can't take advantage of it.