Here's the story on the DV…
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This picture is from an engineering paper on VW's first application of the DV. Notice the flow direction. The designers had two things in mind. When pressure is higher on the lower side of the diaphragm a force is generated that opposes the spring force that holds the valve closed. The outlet pipe goes to the compressor inlet, so pressure on this section of diaphragm is normally slightly lower than atmospheric. The small port on top goes to a pneumatic control signal (In the Audi TT case the EMU can switch the source between just downstream of the throttle (boost or vacuum) or a vacuum reservoir or vented to atmosphere). Under normal conditions it's vented, so for the vast majority of time the diaphragm sees little pressure difference and therefore is unstressed. When the throttle is closed vacuum is applied allowing the pressure from the outlet pipe to push open the DV (there's no such thing as a suction force, it's the higher exterior pressure that is exerting the force). The inlet pipe comes from the compressor outlet. The designers figured if for any reason an overpressure condition should present itself the force on the inlet area of the valve would force it open and thereby protect the motor. The problems started when VW/Audi began selling cars with greater turbo pressure. With this design, the valve would blow open under normal circumstances. So the quick fix (not requiring a DV redesign) was to change the direction of air flow. Very high pressures are now required to open the valve. There are a few downsides to this quick fix: 1) Overboost protection by way of the DV is no longer available. 2) The diaphragm now sees boost pressure whenever it is present, greatly reducing service life. 3) Slower valve lift, in the past when the valve began to open it would show more of the diaphragm to the high pressure side so the valve lift would increase. Now the area ratio is much smaller giving much less lift effect.
Audi uses the same size DV on K03 and K04 cars so don't worry about flow rate. You can continue to use the stock DV but expect a greater frequency of repair. You can switch to an aftermarket DV but don't expect it to be problem free either (based on my experience with Forge and posts I have see on Bailey).
