I'm interested in what Turbosmart's response is to this. Perhaps a defective o-ring? Or perhaps a design flaw? You sound like you know what you're doing. I was actually weighing up getting one of those but might hold off for now.
I would say the plumb back is a good option, it's easier to set up (i.e it doesnt need any setting up at all!) and the recirc noise through the induction sounds just as good IMO.
I've checked over the valve again, and it has been fitted for 6 months, mileage is not mentioned by TS when it comes to servicing the valve. In that time, I have covered 6000 miles.
The piston has been removed, it is of brass construction and one assumes that its machined to match the internal bore of the housing, which is black anodised throughout. This tight interference fit allows it to seal. Spring length has been checked and meets the correct 72mm.
There is plenty of scuffing to the piston wall all around the circumference, which is cause for concern, and another area that could be preventing it from holding boost. Because it also vents to atmosphere, any 'blow by' of pressure around the valve will go straight out the port, and its feasible we have been losing pressure here, as well as the main O- ring on the body.
I will say what Turbosmart have to say about it. I've been more than happy with the valve noise and performance but over time, wear and tear does seem to have taken it's toll.