I'll share this, and it is not to disagree with anything said above *it's all true*. This has just been my experience with a 50/50 "hybrid" BPV.
After a lot of pre-load on the spring and months of tinkering, I have a sequential 50/50 valve working... OK. It's sequential only by having the recirc orifice lower down than the atmospheric vent so under regular driving conditions it (usually) behaves as a recirc valve... but under higher boost it does vent a good portion into the atmosphere.
So the car does run well... but there are still the obvious, but fairly mild side-effects:
-Single but LOUD pop (like, gunshot loud) each shift when WOT. Personally I've grown to like it... but until I had the valve dialed in it was pretty much just embarrassing because it would happen on random, especially during lower gears (i.e. decelerating downtown lol).
-Since the spring is pretty damn stiff, at very low boost levels the valve does not open so there's a bit of a compressor surge (but this is VERY low boost).
What my car does not
have since I was finally able to dial in that I had to deal with for months
-Stalling when pressing the clutch in
-Stalling when trying to reverse
-Uncontrollable popping ALL the time, in pretty much ALL driving conditions
-Idle Issues (idle dips down when pressing clutch, so on).
Personally I'm happy with it now that it's properly dialed in. But that took a long while and it was very embarrassing until I got here.
I have not experienced any performance issues with the current setup because of the BOV. Boost comes on quick during shifts with my current setup (about as quick as my TD-04 did and I have a VF39 now).
I *do* like the sound... I also like the popping and banging/gurgling and all other noises that my car makes including the fireballs that come out of the exhaust... after all this is a wrx. Maybe it's a little immature (the wrx is the opposite of mature, but that's just me
) but I do not believe any of what I'm doing is causing damage to my motor or degrading my performance in any noticeable way.
... and this is a VERY big but:
I understand what my car is doing. I do my own tuning and have plenty of hours dedicated to make the system work in that way and tweak/maintain the valve regularly to keep it operating as it should.
I would not recommend doing this to others because I had all sorts of issues when I first put it on not knowing what I was doing and what the side-effects actually did. I was not able to dial it in until I developed an understanding of tuning as well as the rest of my car's systems and how they work together.