Chayro - I agree cabinet design is more complex, but he is at least asking the question. And I have vast experience I'm willing to share. So by the end, he'll have some knowledge needed to make good decisions. Would I say that Harbeth and Spendor are based on misguided theories? For the most-part, no but purposefully introducing energy storage and uncontrolled resonances to introduce some 'color', absolutely is a misguided approach. Color can be added without all negatives of a resonant box, reality is it costs money and time, and it's easier/cheaper to let the marketing dept try and convince people that it is actually a good thing.
Regarding baffle dimensions, it's the size of the baffle, position of the tweeter on said baffle, and shape of baffle edges (square, chamfer, roundover, and size of chamfer or roundover) that determine the diffraction signature. Typically with less than a 1" roundover there is a 2-3db diffraction dip around 3000hz. This dip is often accounted for in the crossover by adjusting the knee to bring up the diffraction suckout area. That's broad strokes, but gives a good general idea of what's important. Much better than adjusting the crossover to bring up the diffraction area is to build a baffle/cabinet that eliminates the destructive point-source diffraction. If the designer of the speakers you're building says no account was made in the crossover for the diffraction dip, you can incorporate large roundovers as long as you keep the baffle dimensions within an inch of intended. Because the baffle dimensions also impact the boundary reinforcement of the tweeter, which impacts it's bottom end extension. FYI, the area below the woofer on the baffle is of minimal importance. So if you keep the tweeter located where intended on the baffle (center, left, right), baffle width the same, and distance from top edge of the baffle to the tweeter all the same, then you can for instance make it into a floorstander and keep the tweeter response the same. That can have a slight impact on bass, typically means more reinforcement or more bass. But you're only talking 1/2db to 1db at most.
Ptss - you'd be surprised how much energy even small drivers can dump into a cabinet. The magnet systems are extremely efficient, where drivers are much less efficient is coupling to air. But yes, even with smaller drivers cabinet construction can be critical.
