The maximum amount of mobs that will spawn on the server is approximately 0.24 * chunks loaded by default (in vanilla also). "New chunks" are not preferential to mob spawning in any way.
You are right in the sense that if one particular player amasses 0.24 * chunks loaded mobs within his own radius, then no further mobs will spawn on the server. This doesn't seem overly likely however. You are comparing your assumptions on Spigot with a player farming mobs to then just a single player on Vanilla. If you set spawn-range to 8 and compare identical environments betweeen vanilla and spigot I think you will find very little difference.

Then i have been totally mislead by what I've been reading in different forums, and for that i apologize. from what i have heard, spigot uses another algorithm. 

our players are not clustered, they are spread out. and the mobcap is not reached because of spawners. we where at one point 3 players on the server. 2 of the players have optimized mob farms and areas suited for this. the third player stands afk. these are also 3 totally separate locations.

both mobfarms of the 2 first players stops spawning mobs completely. i then moved away from this area, in to an area where mobs should spawn naturally (loading new chunks) no mobs, it was like playing on peaceful setting. this is something i have never experienced when running vanilla server. when i went ahead to check the player standing afk, there were a huge amount of mobs around him and the surrounding caves. thus making me believe he is hugging the total mobcap (still, with only naturally spawned mobs)

if this do not sound like a bug, then again i apologize. as i have stated earlier, i am not so technical that i can point at any code to validate my assumptions. my assumptions where taken from past experience with vanilla, and discussions I've had with people on different forums. 

What Spigot feature are you trying to say is the issue?
The only difference is Vanilla will always spawn mobs in an 8 chunk area around a player, whereas Spigot will allow configuring this value (and defaults it to 4 chunks, and doesn't allow it to go larger than view distance).
By default Spigot will cluster mobs more closely to players as a result of this, but it otherwise doesn't change the limits. You seem to suggest in your ticket that there is some sort of "total mob cap" setting, but this is not the case. Maybe you are referring to spawn limits in bukkit.yml? Well those settings are just configurable values to what is hard coded in Vanilla. Those limits are also not for the entire world, they are scaled according to a Vanilla algorithm: (limit * chunks / 17^2). More players joining and loading more distinct chunks will raise the limit accordingly. Maybe you have players all clustered in the same set of chunks?

I'm not sure where you get this idea that Spigot enables players to "hug the total mob cap" any more than Vanilla - can you point to any code that validates your assumptions? Vanilla has nothing to take into account mob limits either aside from the spawn limits I mentioned earlier.
If you want mobs more widely distributed, then spawn-range is the option you want, but really I cannot tell from your description where/how you think Spigot is different.