Do we really need anything more than 8" woofer and 1" tweeter for medium size room?

The big unaddressed question from the OP is "What constitutes 'relatively clean'".   At 85db in a mid-sized room, most 8" woofers are distorting horribly.  The good news is that the ear isn't terribly sensitive at such low frequencies, so you don't necessarily hear it ...... Until you switch in lower distortion (read larger) drivers to handle the bass.

To get a visual sense of this, visit the subwoofer tests section at htshack.com.  You'll see that a purpose built 8" subwoofer driver (like the Velodyne SPL8) produces vastly more distortion tha a good 12" sub (say Rythmik) at the same frequency/spl.  If you think that tests aren't indicative, I'd suggest you actually try it in you own home.  I did, switching out the little Velos for the Rythmik 12s. Instantly evident improvement.

FWIW 

Bigger Drivers tend to work less harder. Small Drivers work harder.

The Room is very important, smaller drivers disappear easily with not much effort put into the Room acoustics. Larger drivers can disappear just as well but you need to work on the room further.

I like this comparison.   "There's no substitute for cubic inches"
in the automotive field.  So there's also no substitute for woofer size "modifying" Hoffman's Iron Law just a little

If you listen to only certain types of music and never exceed certain volume levels, then I imagine the truncated frequency response, limited dynamic range and high distortion levels of an 8" bass driver really wouldn't present a problem.  If you want to go loud, you'll need more than 8".  If you want to go low, you'll need more than 8".  If you want to go loud and low, then size really does matter.

@gvasale

In absolute terms you are right. There's no way to get high dynamic range and low distortion bass without moving a lot of air, which means driver diameter.  However....

With proper room treatment, placement and tuning a good 6" to 8" woofer has much better bass and dynamic range than most people realize. Also, the lower in response the speaker goes, the more listeners get into trouble with room modes. The quality of the bass output doesn't align with the quality of the mid and treble anymore.

In car terms, what good is power if you can't turn on a dime? :)

Of course, how much one values each of these qualities is very much subjective. If chest thumping is your nirvana, then inches are your salvation.

As always, it's your hard earned money, get what you like!

Best,

Erik