a nice mod in general, and in connection with other boxer/stacker mods even increases throughput a lot more :-) lots of experiments with different combinations to come :-)
in a first test setup i wanted to try it for 4 or 8 blue belts instead of only 3 (one to three stages of splitters easily and symmetrically distribute one belt to 2, 4 or 8 belts), just like you showed on the first picture with a 4-to-1 merger ...

you already wrote that vacuum belts are "3x as fast as express belts", but later you also wrote that "The mod also includes settings to change the speed of the belts", and thus i set the speed to 320 (8*40) :-) :-(

there seems to be a limit in factorio that restricts belt throughput to 1 item per second and per lane, thus 1x60x2=120 items for any two-lane belt, and thus the given "3x blue belt" (3x40=120) would be an upper limit for any belt. if this is correct, it would be useful when you don't only give a minimum and default speed in the mod settings (btw: shouldn't the minimum be >0, eg at least 1 item per second or per minute?), but also a maximum setting of 120 per second (or 7200 per minute).