This car should easily be pushing 400+ whp, so maxing the MAFS would be an issue regardless of a 3" pipe.
Running the fins/vanes is always a good idea, as they stabilize the airflow in the pipe and make for a more stable output voltage.
As far as re-tuning for this MAFS, technically there shouldn't be really that much you need to do. The airflow curve should simply be sfifted by a straight percentage compared with the stock sensor. What you should be able to do is take a log at idle, once you get a steady idle that is and your MAFS voltage stabilizes. Since there will be slight fluctuation, just take an average value over time.
Next, pop in the STi MAFS and do the same thing. It will drop to a lower load cell since its reading less airflow, but you know that it has to be flowing the same amount of air since you're running the same rpm. Just bring in a bit of fuel to bring the AFR up to where you were and again, look at your average voltage.
You take V1/V2 and you'll get a number a bit greater than one. Then you just take the whole airflow curve, multiple every cell by the number you just got and paste it into Option 2 or wherever you want. Car should run exactly the same.
