Of current events, "server clutter" is an issue for:
* Limbus
* BCNMs
* Dynamis
"Layer zone" events like Salvage, Assault, Delve, etc. already run on a pan-world server, so lag in those events is unaffected by whether or not there's a merge. Of the above three events, I have only had to wait on a Limbus zone once in the last two weeks of daily runs. I've shared a BCNM zone with another party once. I haven't been in Dynamis in months, but I'm sure it can stand a doubling of volume as well.
Given what is currently popular, the only thing that might not be able to handle a tripling of players/server would be the Adoulin moghouse zone lines.