On a mechanical throttle blade you want to adhere to TiAL's recommendation for matching spring to manifold vacuum. Since we have a fly-by-wire throttle blade that likes to do various things on its own, this isn't really the case. Someone more knowledgable than myself could shine more light on the subject, but there are moments when the throttle blade does not open up entirely immediately after releasing the accelerator. You also have to consider manifold vacuum levels are rarely actually measured and people are just guessing, crossing their fingers when they install the BOV. When you run the AC, your manifold vacuum goes down as well, check it for yourself. I've seen my car only pulling 6 psi of vacuum in the morning with AC enabled, which is a far cry from the 18-20 inches of mercury that people state online.
What this ultimately means, if your spring pressure rating is even slightly stiffer than the amount of vacuum your engine is producing at the instant, then you're putting unnecessary stress on turbo. Being too stiff has negative effects. Being too soft.....? Well nobody can tell me what negative effects that has because there virtually are none in most cases except for some made up shit they have decided in their head. Just run a spring that is one setting too soft per the internet experts and you're doing the turbo a favor.
