ok, so this is for the many questions regarding the use of a dv vs bov on your turbo vw motors.
First off, what does the DV do?
When you hit the gas your throttle plate opens allowing air into the engine. The turbo is in the intake stream and compresses the air and forces it in to the engine at a certain pressure. When you need to shift and let off the throttle, throttle plate closes, and the turbo is still spinning. The turbo still spinning is a good thing because it will have less work to catch back up when you reapply the throttle, but... a spinning turbo means it is still compressing air and the throttle plate is closed so there is only one way for the compressed air to get out, back the way it came in. Well this is where the Diverter Valve comes into play. The diverter valve diverts the air that would have gone back through the turbo causing it to stop spinning in one direction and spin backwards, to go back into the intake stream pre-turbo so you recycle that air. Now since your Volkswagen is tuned to run the air back into the system after the MAF sensor the MAF is calculating based on a certain charge still being in circulation when the throttle plate re-opens. This is why using a DV is most efficent on vw's motors, rather then a BOV which vents to the atmosphere. If you were to put an aftermarket Blow-Off Valve on the system that air that is being calculated for just blew out into the atmosphere. This causes the computer to still inject fuel metered for the air that was in the system already as well as the new air coming through the intake. With an aftermarket blow off valve you will get a fun turbo sound but your engine will run a little richer. So basically you need to pick performance or sound?
Benefits of an aftermarket DV:
Your stock valve in your car regulates your turbo power. At idle, your valve is open from vacuum pressure. When you hit the throttle, the diverter valve begins to close. When fully closed, turbo boost is allowed to be delivered to your engine. When you lift the throttle, the boost pressure is released from the valve into your air cleaner box. Inadequate/defective valves have extended lag, loss of boost, sluggish throttle response. A aftermarket diverter valve can clear up the previously mentioned problems. Also, if you have a chip, you need an aftermarket valve now. Chips simply kill stock diverter valves.
Stock oem DV:
Here's a common upgrade for an aftermarket DV, Forge DV:
here's a solution if you want both performance and sound with your 1.8T motor. It's called the splitter by forge, it does both recirculate to keep the performance, but it also vents some air to the atmosphere, thus actin like a normal BOV.
Here's some common BOV's *i dont recommend, but its your car*
Also here's another performance upgrade you can do with your DV. It's a relocation kit. By relocating the valve in this way there are two benefits. Firstly, due to operating in lower temperatures the longevity of the valve is increased. Secondly, after relocation, the air that is recirculated back into the intake tract to the turbo will be cooler. Cooler air has a greater density. The greater the density of the charge mix in the cylinder the more power generated on combustion. Here's a kit from forge.
Forge Relocation Kit:
Now, all of these products can be found at normal vw's parts stores online. Hopefully this helps some questions about DV's vs BOV's.