Heat, humidity, CO2, odor and wind stress can indeed be taken care of in other ways. Air quality on the other hand might become a challenge. For small recreational environments it is not necessary and despite of that it could be usefull at later stages, let me explain further.
Your Softbox 550 should normally be reducing the sound, potentially you are running it on max performance, which might not be necessary. Based on some of the dimensions given by you, I assume you need around 36 cubic meter minute = 1 * 1.2 * 2 * 60 / 4, which is not that much (I assume 2m height, divide by 4 as refresh rate is only needed once every 15 minutes). So your system of 500m3 there only needs to run at 5-10% at max. Depending on the location of your tent, the climate control could be easier to maintain with the system during hot summer days.
It can also become really usefull for small areas once your plants hits the budding, flowering and ripening phase, as I noticed for my own growths that these are the phases where the tent moisture levels rise the most.
Some additional tips:
- Keep in mind that the air intake fan should only have one quarter of the strength of your extractor fan
- In your case it would be good for temperature control for your HID lamp(s) lifespan as well
- If stealth grow is the main issue, consider using timers to run during daytime only, rubber pads to catch annoying vibrations, additional acoustic foam, etc.
Have fun and enjoy your growths!