As far as I know, downpipes will net you more of a gain than the midpipes.
On your model, being an N55, you only have catalytic converters on the downpipes. A lot of the N54 guys buy N55 midpipes to remove our cats as we have cats on both the DPs and MPs.
Remove the cats on the downpipes or getting them swapped for high flow race cats and increasing the diameter or the pipes will improve the flow of exhaust and will also increase the sound of the turbo however if you need to smog your car every 2 years I think you're SOL unfortunately. You will not pass emissions tests with your cats removed or modified.
In regards to the mids, I think that seeing as you don't have cats on the mids anyway you won't see a significant gain in performance by changing them to aftermarket pipes. With removal or changing to the resonators with aftermarket midpipes you will change the sound of the exhaust but it shouldn't affect emissions.
Hope that helps. Good luck!
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