Your right, you don't want turbulance in the pipe but, as you say, you will always get some. So, you should try to keep this to a minimum. To do this, the idea design of pipe (for internal wastegate systems) would be to have a seperate wastegate pipe, that seperates the two gaseflows (wastegate and exhaust) at the exit of the turbo. This would then rejoin the exhaust dump pipe as far away from the exit as possible, within reason and so as not to let the gas temperature drop too much. This will normally be just where the exhaust pipe goes under the engine. This would rejoin with the ideal angle so that the the two pipes are practically parallel. I.e. so the wastegate gas enters the pipe, with the flow of the exhaust gas, so as to disturb the flow as little as possible.
I hope that rambling made some kind of sense. Do some searchs on the web, there are plenty of people out there making this pipes for various vehicles.
As for the the 200 sx, it would more than likely be open neck but, the wastegate and exhaust paths would be slightly seperated. Slight contradiction there but, what I described above is a true 'twin' dumppipe.
