I think your guess is not bad
but split farming is not 4 times as efficient as doing bounties solo. the horadric chests are only a small bonus compared to to drops you have when doing the bounties.
what does a horadric chest equal to, 2 or 3 elite pack drops? how many elites do you kill doing 5 bounties?
I think the overall efficiency bonus for doint a split bounty run would be about the same as a 4 player rift run, which I'd place like you at about 2 or 2.5
to me its absolutely clear that you dont get anything "free" with split bounties, surely there is a bonus for doing things in a game with 3 other players, but its the same with doing rifts, boss runs or whatever, in a group.
Your math is off. the time it takes to run one bounty on normal isn't affected by the number of players. I can solo clear the bounty in the same time my full group can because the monsters are so weak that the only element of effort is the time it takes to get from mission start to mission complete.
Instead of inputting clear time and outputting drops earned during bounty + 1 cache, you're inputting the same clear time and outputting drops +1 +1 +1. It is clearly the most efficient way to use 4 people and completely evades the difficulty vs reward system that most games use as a basis for giving players a sense of achievement.
Blizzard just needs to make it so that only 1 bounty can be active at a time, and if you complete the reward before activating The next bounty, that bounty is removed from the reward pool and you can't get q cache for it. It wouldn't make it impossible to duck the system and do things like have one person wait at the cursed chest or event or whatever but it would at least discourage people by being a pain in the ass to do.