My question would be what do you mean by sleeper? Some people like to make their car look as if it is a non turbo model. Others like the look of their car and just want to add performance with out being a "ricer". So what are you looking for?
Depending on who you plan to have tune your car I would say a re-flash is cheaper but not always better. The AccessPort will allow you to change from stage 1 to stage 2 with out having to buy another re-flashed. I know most people spend anywhere from 300-500 on a flash. So you get an ECU flash and then down the road add more mods(intake, turbo inlet, downpipe) you will need to get retuned and be at or above the cost of an AccessPort. The AP comes with multiple stages built in (stage 1 and stage 2, and 91 or 93 octane) and they are proven to make significant power gains throughout all RPM's - all the while still giving you the ability to return to 100% stock.
Cobb does make an awesome downpipe, but for the "sleeper" I imagine, an accessport would be my first purchase.