Subarus TPS's have to be adjusted when installing. The use of a scan tool with data capabilities will do the trick. Key On Engine Off (KOEO) it should have a reading of .70 volts with your foot OFF the gas petal. Just installing the new one where the old one was will get you close but they need to be dead on for everything to work right. You'll need a better scan tool as most will only show TPS % and it's better to have the volt data. GL
Speed Density systems are some what better for turbo engines as the MAF sensors will create a flow issue to the turbo unless a larger higher flow rated MAF can be found for the application. Plus it's one less thing you don't have to worry about fabing some piping to and from and it looks cleaner w/o them.
What they lack verse a MAF sensor is the ability to compensate for temp and elevation changes accurately. A MAF sensor measures the air flow in real time and can adjust where SD can only make changes from the preprogrammed values.
I ran a MAF Translator in my old Supra, which is a tuning system that changes from MAF to SD. The biggest headache was when I tuned it in spring it got warmer and more humid it would run like crap so I would have to retune it. Then again weather changed I would have to retune it. Being a piggyback system was a downfall in itself. Most standalone ECUs like AEM or Pro-Efi are better suited for using a SD system.