All the CDV valve dose is reduce the internal size of the clutch line, thus limiting the amount of fluid that is able to pass though, as deturmined to be the best rate by BMW. It actually makes the clutch slip slightly, and causes additional wear, and keeps anyone from shifting faster then they deturmined.
All you have to do is remove the valve and bolt the line back together, then re-bleed the system. You can also pull the CDV out and simply drill the hole out to the size of the clutch line and then put it back in. The one Riss and others sell are just stock units that have been drilled out - anyone can do it. This also keeps things looking stock so if anything clutch-wise goes wrong BMW can't see it was modified.
It defintely makes a difference, I did it on my old E46 M3 and will be doing it on my new E92 shortly!
Hope this helps...