You may be wrong on the sound-stage consideration. The factory tweeter location is right where it should be for those directional high frequency sounds. The midrange speakers, on top of the dash, reflect their sounds right back at you from the best location possible.

The midbasses in the doors are just that: bass speakers. Their sound is not directional so it does not matter how they are pointed.

I do not believe you can improve on BMW's speaker location.

You can improve on the actual speakers. BSW offers direct replacement designs that I can tell you are very much better than stock. The highs and midranges are really superior and I don't think there is a way to do much better at all.

Even if you get real spendy and get the very most expensive component speaker system (the kind where you get a 6.75" midrange, matching separate tweeter and dedicated cross-over as a kit) you probably will get some vanishingly small quality difference, but lose it to BMW/BSWs in terms of mounting location.

You will lose your soundstage, not enhance it. This would be especially true for the two-way speakers you are proposing. That is simply a step down.

I believe that you will be better off in every way if you go for the BSW system along with the biggest and best sub you can make room for and the amp to power it.

You could maybe do a little better substituting the BSW mid basses, front and rear doors, with more expensive JLs, Alpines, etc. I can tell you my Alpines are far better than stock, but I have not compared them with BSWs.

You could talk to BSW about their new amp. That would I am sure be an improvement, but the little stock amp runs efficient speakers pretty well.

I think you can do very, very well without spending a fortune or modifying a bunch of stuff. The BMW is rightfully condemned for their cheapo speakers, and no sub, but they did their homework on location and cross-overs. I would take advantage of that.

On another car I would give completely different advice.