I find that hard to believe. A blue belt with size 4 stacking will move 160 items/sec, with 80 items per lane. That's a lot of items!On their own it won't be enough to deal with belt problems
I also find this hard to believe. A flat belt can be saturated with 3 upgraded fast inserters per lane. Multiplying the belt capacity and inserter capacity by 4 (inserter 3 => stack inserter 12) leaves these values the same. It's not a big deal to use 6 stack inserters to super saturate a belt. That's one side of a train unloading station, with each train car saturating a super belt.inventory-to-belt transfer speed of Inserters are also a main source of the limitations at hand even with Stack Inserters
The small question is if such high capacity can feed a module/beacon setup. I think they can. The big question is if it players will use those stronger belts in their builds. That's a bit too difficult for me to know!
Edit: I don't think belt stacking has any benefit beyond low tech items. It's hard enough to load a belt with red circuits, never mind trying to saturate a rocket fuel belt. Oh well.