Hannu,
I posted something like that some time ago using a 3 bladed savinous to catch the air internally.
It will definately help to capture more of the incomming power. I don't think adding more blades or vanes will help though. It may tend to disrupt to much incomming air and add to the drag on the up wind side. With a 3 blade arrangement you have one on it's way out and one starting on the downwind side and the drag of 1 blade on the upwind.
My efforts with the Lenz turbine was to create as much low pressure behind the mill as possible so the incomming air will tend to carry the blade farther around as the incomming air tries to fill the void. I highly doubt I'm getting the full effect from this but it does make a reasonably efficient VAWT, at least in the test models I've built so far. Hopefully I'll have some time this year to see how a larger unit fairs. By the way things are going right now it may not be until September when I get caught up with everything... we'll see...
Have Fun!
Windstuff Ed