I've never looked into it. The limit of 300 is there for a very good reason. Unless you've got NASA level processing power just sitting around, it's worthless. And if you intend to let other people play your scenario, it's also worthless. Basically you'll be stuck with a scenario that only you and a very select group of people with Athlon 4+'s lying around will be able to play, assuming you can.
Anyway, try things like adding extra settlements. I think there's a code you can modifiy, but that will also limit your scenario to you and the one or two insanely bored people out there who want to mess around with their game files to play your scenario.
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