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

You cannot predict what you will experience when sitting back in your chair from specs such as these.  You MUST listen to the speakers and preferably in your home.  Your example is 0.5 in difference from the same mfg, but from a different generation.  You wouldn't expect any difference here but the newer generation may be far superior to the older design.  The factors that contribute to a speaker's sound is extensive as others have mentioned.  I have Platinum Audio Quattros that have four 4.5 in drivers, two crossed for mids and two for bass in a nice slim cabinet.  No way this speaker can extend low.....right??  Well, they would shock you on how effectively they can play down in the 30s and loud too!  I tried out the Focal 1038Be2 with three 7 in all dedicated to the low end and in my room they were weaker in the bass compared to the Quattros.  Platinum Audio speakers were designed by Phil Jones, who is a bassist and has a great love for good bass from his speakers.  This goes back to all the other factors and how he designed his speakers.  Now I have Wilson Audio Sophias with a 10 inch driver in each.  They far surpassed my Quattros in their lower extension and have become my new speaker.  Let's do the math and measure the amount of air displaced from the cones surface area.

Platinums(two 4.5in) 32 sq inches
Focal(three 7in) 115 sq inches
Wilson(one 10in) 78 sq inches

This would imply a different scorecard for sure!

The "speed" a driver is capable of is indicted by its frequency response curve. A 15" driver pushing 100 Hz is every bit as fast as a 6" driver pushing 100 Hz and if the motor is designed correctly the transient response can be just as good or better. This concept of "speed" or fastness need to be thrown out.

6-8" woofers can perform admirably down to 50 Hz or so. You can force them to go lower but forget about energetic bass below 50 Hz. Small woofers are being used to support the tower concept that the ladies like. Using multiples helps but none of these speakers do well below 50 Hz. If you think they do it means to me that you have not heard a system that accurately produces low bass. I use a subwoofer array that uses 4 12" drivers with a total of 8000 watts in a 16 X 30 foot room. If I play a 10 Hz sine wave the entire house shakes and buzzes. You can actually see some items shaking. Everything upstairs and in the shop is singing. If I turn it up loud enough your vision blurs. Little woofers simply can not do this, physically impossible. What they really are is midbass drivers which they do wonderfully well. For the best performance you cross over to subwoofers at 80 Hz. If you use a symmetrical array around the main speakers you can cross higher. Good midbass drivers can run nicely up to 2500 Hz or so and there are many tweeters that will easily go down to 2500 Hz. So, you really only need a two way speaker and subwoofers. 
The minimum is four 10" subwoofer drivers or two 12" subwoofer drivers.
Subwoofer drivers have a hard time making it to 500 Hz for design reasons. 

Why not fill in the low end with a pair of REL S2 SHO subwoofers.  You can dial them so they don't over power the bass and with their high level connection they will act more like woofers.  Be curious to hear what this group thinks about this.  These really helped to broaden the sound stage.  Trick is to make sure you do not over power the bass.  I noticed my mid frequencies and higher frequencies seemed to clean up.

I've been using a pair of Velodyne HGS-15s crossed at 40 Hz with KEF Reference 1s.  Today I raised the crossover to 80 Hz, still using a 24 dB/octave slope, and ran acoustic room correction.  I was surprised how much that enlarged the sound stage.  My concern about the setup being suitable for large orchestrations vanished.

db

I don’t have a lot of experience with different speakers, but I can tell you there is a big difference in bass between my current Klipsch Cornwall IV’s and my Goldenear Triton 5’s. On paper the Tritons went deeper into the 20’s while the Cornwalls only go down to 38 however the sound is bigger. When I added two JL Audio Dominion D108 Subs with the Goldenears the Sound was great but still not as powerful as my Cornwalls. I’m actually thinking of adding my Subs back into my system to see how they’d sound with the Klipsch. But is it useless to use two 8 inch 500 watt subs with big 15 inch woofers? So in my experience albeit limited bigger woofers can provide bigger sound but not always deeper bass.