Bass Driver Size - how much better is an extra inch?

There is no replacement for displacement. Axiomatic in the car world, and as well should be for producing bass.

Of course, on both fronts there ARE replacements for displacement. But slapping on a turbo or just increasing xmax isn't a true replacement Hence the non-linear price increases for well executed power at smaller displacements.

Much value being offered herein albeit cryptic. 

I believe it's largely fashion.

Not everyone wants to, or is even able to accommodate speakers with huge 15 inch bass units like me. 

Drive unit size is certainly a contentious subject with various arguments for and against large and small drivers.

It's certainly not a question of bass content either as some small units can dig lower than some larger ones. Depending upon factors such design (greater number of long throw units) and cabinet loading.

However there do seem to be differences, size does matter.

Permit me a gross generalisation if you will, but I'd say that smaller units sound noticeably quicker with transients.

On the other hand, larger units tend to have a far greater sense of ease as they go about their work.

The 15 inch Dual Concentrics on my Tannoys are easier on the ear than the slimline Revolution 3s that they replaced, if perhaps surprisingly a little lighter in the bass.

If I had to chose, and if I wanted a more relaxing sound, I'd take the second option. 

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

Right, most people (women) do not want big speakers. 

Assuming proper driver design larger woofers can go lower and have less distortion because they do not have to move as far to create the equivalent output of the smaller driver. There is a limit. As the woofers start getting over 18 inches (some say 15") it gets harder to control the motion of the cone creating more distortion not to mention the size of the enclosure gets prohibitive for home use. I think 12 and 15 inch woofers do fine in the home environment. There are 15" drivers now that will work in a 1.5 cubic foot enclosure which is darn small.

Many people notice that for some reason the midrange opens up when subwoofers are added. The low bass gives you a different midrange perspective. If a two way crossover is used distortion in the main speaker is reduced. If a lower order crossover is use enough midrange may come from the sub to change the perspective but I notice this even with steep slopes and we are talking about 10th order. The problem with steeps slopes is the crossover becomes more obvious. The compromise is 4th and 6th order which works great at least in my case.