"Ideally would be BL 5 front, In the middle BL 9 and at the rear BL 3. This was the original idea. I have looked at some posts but there are no people with such a setup? Even the cinema set at the dealer doesn' t have a setup 7.0 I believe."
I experienced exactly this setup in a Beostore near Copenhagen. It`s actually a 7.1 setup, because the LFE channel (.1) will be added to the front BL5s. So you don`t need an extra sub. You don`t have a "middle" channel. BL9s reproduce the rear channels, also called "surround" in 5.1 setups, used for athmos and sfx. BL3s are for the "back" channels, mainly used for athmos.
"Isn't it just a waste of money to put in the BL 9 in the middle? Shouldnt I just upgrade the BL 9 to BL 5?"
I wouldn`t call it "decadent". I would call it "consequent". You want matching speakers in a stereo setup, because you don`t want to experience tonal differences, especially in moving sounds like driving cars, flying helicopters etc.. And you don`t want these differences between - for example - front-left and rear-left. BL5 and BL3 sound different, especially in bass reproduction - we all know this. So the intention should be to build a setup with matching speakers. If you have BL5s in front, get BL9 or BL3 with BL2 for rear channels. Or again BL5 for rear...
BTW the 7.1 setup sounds awesome, even with 5.1 movies there seemed to be a difference between rear- and back channels....(?)
Stefan