TL;DR
In the 0.16.20 update, the Factorio developers fixed some of the problems with priority splitter throughput, but some problems still remain.(I suspect that the "input priority" feature is working as intended, which is why I posted this as a Suggestion instead of a Bug Report.)
What ?
I want to be able to build this 5-to-6-belt splitter: Unfortunately, this still doesn't work because of some problems with the "input priority" feature.To see the problem, let's look at a small example. In the example below, the top belt should not have iron plates leaking onto it: The misbehaving splitter is the splitter on the right. It is receiving two blue belts of input (one iron and one copper). The iron input is prioritized using the "input priority feature". It has two outputs: one blue belt and one YELLOW belt, and therefore it should be that all of the iron plates pass through the splitter. (The yellow belt is just simulating some part of my factory that demands less than a full blue belt of materials.)
However, sometimes the iron fails to pass through the splitter, causing the iron plates to "back up", i.e. they don't have full throughput. To make this more obvious, I have included the splitter on the left (this splitter is working properly ). If the iron plates were not backing up, then there would be no leakage onto the top-most belt.