You dont go only by the size of the rad. The X40/X60 also has larger tubes, improved cold plate (Not the same as the 620/920), also the FPi (Fins per Inch) of the rad.
Plate heat transfer is important yes, but from what it looks like all re-badged asetek resellers will be using the same updated asetek copper plates. I doubt this one will be any different than the antec v4's internally.
Tubes diameter at this length makes NO difference. It takes some serious tube length to make a significant difference. (Check out http://martinsliquidlab.org, proven there and many other sites.)
Just like any well designed heat sink, the most important component for cooling is surface cooling area, that's everything between the tanks for a rad, NOT the tanks. Just about every vendor today sells rads with good flow these days. NZXT was smart going with 140mm, Most people who would consider solutions like these most likely has or will have a case that will fit a single 140mm rad. Right off the bat there will be more surface area having a 140 over a 120 at same depths.
If they went 12 MM deeper on a the single rad, They would have matched the surface area of an h100 (give or take a couple of %) and significantly beat all other vendor solutions. that's a big deal from a marketing standpoint considering it is MUCH easier installing in a single rad over a double in most current cases. very appealing if the price makes sense.
The x60 doesn’t need to be deeper than 27 mm (although if it was 10 to 15 mm deeper something tells me that it would have potential to hold two pumps, cpu and vid. THAT would be T!T$ ). The X60 already offers around 28% more surface area than the H100. If asetek improved their transfer plate, then the X6O will be a beast, and should be untouchable with cooling and noise. Again, that's if Asetek improved overall design. Also asetek might be close to a threshold for the pump with that kind of volume, just an assumption; the rad already has volume that is close to an aquacool dual 120 ut45, which is significant.
Anyway, I have nothing against NZXT, I love the fact that they are creative and try to be different. I've bought their products and will continue to do so, (I can't get the x60 fast enough for the new case, ship me one already ) all I'm saying is I find it hard to believe that their marketing engine "overlooked" something so obvious and I'm wondering if they have plans for a deeper single 140mm rad solution. Somebody’s going to make it; it might as well be them as they already broke the mold.