You can SO do house clearing/CQB type stuff with a full length rifle. I've practiced it with my 21" FAL on occasion; the trick is to strip all of the extras off, like the scope mount, the bipod, etc, and bring down the weight.
That 5" more barrel can make a difference in close confines, but with a little practice, you can work around it.
Though, you're probably not going to be doing a lot of "house clearing" with your rifle anyway. That is much better suited to the shotgun, which even with a 20" tube is still managable.
The only 20" AKs I've seen are the VEPR IIs and the Saigas. My friend has a Hesse (it works okay, despite) Saiga conversion with a 20" barrel, and with the standard Saiga handguard on there it looks pretty good, actually.
The VEPR IIs from Robinson look pretty cool, too, though I liked them better with the old front sight post than with the new one. Robinson told me in an email that all new VEPR IIs have the traditional AK front sight assembly.
This is the VEPR II .223 with old style sight tower.
VEPR II 7.62x39mm with traditional AK front sight tower.