Custom Box for Alpine Type X 12" I'm building one, got some questions...Thank You...? - does a custom box make a difference for your subs

Ok, so I just bought a new Alpine Type X 12 "sub and I have the manual that comes with all the correct information on the --
Affirms that the ventilation of the box is best --
Case Exterior Dimensions: 14.5 cm x 16.5 cm x 17.5 "
Gross volume: 1.8 m3
Slot-vent area: 19.5in2 (13 "x 1.5")
Ventilation Slot Length: 27IN
Wind 0.46ft3 shift
Net volume internal: 1.15ft3
27Hz, 5.8dB, 32Hz

but I had a bigger box like, because it seems preferable to a place in my ...... filled trunk to build an array of factory specifications are often very specific design sub-committee, but the most important part is the cubic volume of air in the room takes more than the dimension, so if you see pictures on YouTube, sometimes larger, It is up to the strains of some boxes do not correspond exactly to the specifications and the manufacturer has to make sacrifices breadth to the Subcommittee to carry out his car to other factors, as some people think tha t bigger = better speakers and as an idiot box which is too large, and the driverER preform poorly if too much air to move the same thing with a box that is too small ...

But then what about this picture I found on eBay for the type of x (click link)

% Http: / / cgi.ebay.com / was / eBayISAPI.dll? ViewItem & item = 400035461807 & ssPageName MERCOSI_VI_ROSI_PR4_PCN_BIX_Stores & refitem = = = 400,032,427,318 & itemcount refwidgetloc = 4 & & closed_view_item refwidgettype osi_widget & _trksid = = = p284.m185 & _trkparms algo 3DSI% 26its% 3DI% 252BIA% 26itu% 3dia% 252BUCI% 26otn % 3D4% 26ps% 3D41

with dimensions of
Dimensions (W x H x D): 30 "x 13.5" x 16 "
2.0 cu. Net volume ft

So what is the difference in performance with this case and suggests that the Alpine Guide ...?