is the customized box worth it?

^^^ True, but leaks are the most obvious and amaturish conditions that make a box a poor box.  Getting past that, the biggest killer of good sound is using a bandpass or vented box that is premade for a variety of uses.  Chances are close to 100% that it will have a negative effect on the sound of your sub.  Those must be custom-made by a pro who knows not only how to design and build such a box but also has a good sense of the abilities of the particular subwoofer.

The other great killer of sound is wrong air volume in the enclosure for the sub being used.  While a 1.25, for example, may work well in a sealed box for one sub, it will make a sub sound less than it should with another.....even if both manufacturers recommend that box size!  Because consider this:  if you were producing a product that you hope to appeal to a large group of buyers of every level of competence,  how would you market your subwoorer?  You would say to put it in a box that has a size that is easily found in prefab choices.  Or that has a size that fits most vehicle applications.  It may work out to be the best solution or it may not.