Tobus- typically box pleat kilts do not overlap. Standard box pleats make for a roughly 4-yard kilt, and double-box pleats make for a roughly 6-yard kilt. The goal in a box pleated kilt isn't to add more material, really (not that that's the goal in a knife pleated kilt, but you catch my drift).

With a knife pleat, you're right, overlapping more will add more material, thou you're mostly left to the tartan itself to determine the overlap/pleat depth.

Now, in the other type of box pleat - a military box pleat - you do get overlap and use roughly the same as a knife pleat, since this style is basically a knife pleat turned back.

You're pretty spot on with your thoughts on drape/fan. More material will give more swish.