Take a look at a fairly similar situation. Shards of Dalaya (custom EQ freeshard using up to Titanium expansions) has a comparable population. It also has 13 raid tiers with more raid mobs in most of each tiers than all of current P99's zones combined.
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There are also 6-man encounters, 12-man encounters, and adepts (basically lowbie raids). I was on there for a year or two raiding T1-T10 via a couple guilds and rarely encountered issues with other guilds raiding a few hours three nights a week. There were some issues sure, but like I said, fairly rare. Am I saying this place is heaven? No. Am I saying P99 should try to emulate a custom server and add custom content? Obviously not as it defeats the purpose of the project in the first place. I'm just trying to communicate an idea of the amount of raid content it seems to take to make 500-1000 people happy and consistently raiding. Now, take that raid content, divide it by 13 and sit with it for three years.
It's amazing you people get any raiding done at all without everyone's dicks going into everyone's butts. I'm lurking until Velious. Not sure it will change anything though, even doubling the raid content.