Forge DV's cost a fortune and require maintenance, ie: greasing of the piston, otherwise they can fail to seal = boost leak. They may look pretty, but unless you are going to show off your engine bay on a concourse somewhere, why bother with the aesthetics?
I'm running over 400bhp on 710N's and I know that there are some RS4's running higher boost still using these. They cost about £40-50 delivered for a pair and they'll last for a few years.
No brainer IMHO.
Service intervals will vary based on how often the car is driven and environmental conditions, for example, a valve on a car in an extremely cold part of the world will require more frequent service than one in a temperate climate, as will a DV used in a very dusty or sandy location.
My usual recommendation is to test the valve once or twice a year and service as needed, which might not be but every year or three.
There is a PDF in the media section of our site that details the test and the service procedure. I am unable to post a link due to insufficient posts, look for "valve service guide".
That being said, the servicing is to ensure proper operation of the valve for what will essentially be the lifetime of the car. The OEM valves fail on both of these points.
Just about any valve will "hold" an extremely high amount of pressure but nearly all OEM valves (and many poorly designed uprated valves) will close very slowly on boost onset, which hurts response, and many that are softly sprung run the risk of leaking boost as maximum boost pressure is reached due to a pressure differential between what is acting upon the discharge side of the valve (charge pipes) and the top of the valve (which gets its reference from the intake manifold). This is caused by pressure drop through the intercooler and other factors.
The stock valve suffers from both of these failings. Remember that turbo-charges are a feedback system. The pressure control mechanisms (N75, wastegate, etc) will try to reach a determined amount of pressure even if there is a leak. In this case the leak is the factory diverter valve. If the leak is severe enough there will be an apparent increase in boost pressure, although in reality it is just a decrease in the amount that is lost. If the leak is less severe peak boost pressures will remain the same but pressure curves will be much different as will the shaft speeds seen by the turbos. Which will prolong engine life and result in cooler air inlet temperatures, which are both desired results.
Also, when the factory valves fail, as they most likely will, the few days of drving with such a large leak will absolutely wreck havoc on the turbine bearings as the turbos spin to astronomical speeds to try and pressurize the intake pipes in vain as air leaks from the ruptured valves.
@DPM: I'm surprised that you could hear your DV losing boost on the motorway with the wind and road noise, especially seeing as how the DV's are pretty much silent.
They become more audible as the amount of air that they are failing to contain increases.
It's possible that if you were running high boost you managed to outperform your 710N on an S3, but remember that the boost pressure is shared over two DV's on an S4. I'm running peak 25psi with no problems on 710N's, ie: 12.5psi per DV.
This is incorrect. The volume that is vented by each is half what it would be with a single valve but the pressure is the same.
Sort of defeats the idea of a January Special 
