Oh, they cost a fair bit then. I am not much familiar with ICE and have been around under 1.5 Lac setups only. We are all here to learn and share. So this is my personal view from whatever experiece I have had and it is perfectly alright if you accept it or not.
In hatchbacks, due to their structure and body frame, SQ needs can be met with a set of high-end components at the front (like you have) and an SQ oriented subwoofer at the back, with proper tuning. I agree to many speakers spoil the soundstage but 4 speaker + 2 tweeter + 1 subwoofer was the best setting I could find in sedans with the limited knowledge I have. Due to a hatchback's design, more of bass is gained inside the cabin than what is actually produced. In sedans, I found this to be opposite, the bass produced inside the boot and transferred in the cabin was very different, and was equal only in some setups. The bass levels in the cabin are much lower. This I am telling after auditioning a 120.9w Infinity playing in my Swift and then in the installer's SX4. SQ setup is not about getting good vocals or good bass, it is a blend which is needed. For a good SQ setup, sound imaging is very important and although a major bit of it is altered by proper tuning, the other primary half depends on the equipment and their position in the cabin. In a Fortuner with DLS components powered by a Focal Solid 4, I had the chance to audition with the subwoofer, with rear components and with front components only. The final setting which the owner liked was 2 pairs of components (tweeters at equal height, flush mounted) and a 12" subwoofer. The rear seat occupants (me and my friend) could easily tell the difference the rear components made. For the driver and co-driver the difference was a marginally better sound (clarity) but it was a difference between good fuller sounding audio and a front-heavy sounding setup. IMO rear fills get more important as cabin size (and thus volume) increases. I agree with your posts except the dual subwoofer part, if we are talking about decent 12"ers here. My MM1240 Polk feels sufficient for low-mid volumes and blends well with the front components. As the volume goes up, the subwoofer definitely starts to overpower the front stage. One subwoofer with a respectable power handling, high BL and excursion will be enough for an SQ setup. I have also been in a Punto Evo with 2 L7s kicking at the back and believe me, they were literally breaking the car with every bass kick. Let alone the IRBM, even the ORVMs were shouting for their lives on bass drops! Though bass needs are different for everyone, 2 subwoofers sound like an overkill to me unless you got some serious tuning equipment (to make them hit oy when needed) and powerful front stage to cope with the bass.
While I agree that 6x9 coaxials do make the setup rear-heavy but I think the round coaxials balance the sound very well and are more apt as rear fills for those having decent components and a subwoofer already. Opposite to the case of subwoofers, the presence of rear fills is felt and not their absence. This I am telling after my own experience with JBL 6x9s and the current Hertz 6.5" in my tiny little Swift. In sedans, the absence of rear fills will be felt more than in the hatches. This is not evident much at the driver and co-driver position but is evident in the rear bench. Maybe AKHAN can himself experiment during his install (by auditioning with & without rear fills at the rear seat) and let us know his views. I think you have heard many SQ setups in sedans also, and as you say, I definitely need to do some more auditions myself to put down my views.
Let us not hijack AKHAN's thread and one last thing, you need an amplifier or equaliser for those speakers sir!