More important than a downpipe is to ensure the exhaust behind the downpipe is good.
My car picked up .4 and 4 mph going from a crappy exhaust to a good exhaust, but in back to back testing only picked up .05 and 1.5 mph switching from a SS Stock Replacement 2.5"/ported elbow and a 3" Terry Houston DP. I bought the elbow and pipe used for a $120 (about the going rate). This was well into the 11's, too.
A ported elbow and the stock downpipe won't be too much of a restriction if you still have the stock turbo/injectors and intercooler. A good downpipe will be faster, but as I found, not by much.