Why stack size not calculated in line throughput?

I didn't come up with a good way to properly implement drill counting, speed and productivity, and checks for belt merging. It's not exactly simple as multiplying the throughput cap by stacking size, is it?

Stack size proportionally increase belt throughput. For example transport belt 15 items/s with 3 stacks have total throughput 45 items/s. All drills output in stacks if it researched.

Not all drills. Only those with enabled drops_full_belt_stacks Factorio API
Electric mining drills don't have it enabled. Big mining drills do.

Oh, I see. Its pY mod make all drills out in stacks, even burner drill.
But can you make alternative output in brackets throughput using stacks?

The mod API in control stage was lacking that drops_full_belt_stacks read so I made a request for Wube to add it but it's coming in Factorio 2.1
https://forums.factorio.com/viewtopic.php?t=132055