To answer your question "
how much better is an extra inch?
" the answer is, not much -- unless you have a very large space that your present speakers fail to fill.If that is the case, a 8' woofer will give you a slightly bigger image and will be somewhat louder in the bass (i.e. they do move more air!).
Of course, at equal SQ (speed, accuracy, etc) large drivers are much more expensive... there are other considerations as well
Of course, at equal SQ (speed, accuracy, etc) large drivers are much more expensive... there are other considerations as well