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

My speakers have two 6" TL drivers per side, and a ribbon, and I augment them with an EQ'd 15" sub in my 24' x 20' room--but that's a personal choice. I listen mostly to rock, pop, and old-school R&B and country, and my combination of speakers/sub delivers a decent amount of impact in my treated room. But I think the choice depends a lot on room size and listening preferences.

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.