I am playing with he idea of running a Freeport game with a focus on cults and corruption. Would using only 15 points for attributes help reinforce the tone, or do you think that is necessary? Also, if any one feels like throwing in their two cents about a Freeport campaign, that would be appreciated.
I ran the original Freeport trilogy back in the 3.5 day, and most of Green Ronin's published adventures for it.
15 point buy should be fine, although this will affect the tone less than the access to magic and NPCs they face.
Generally speaking, the Freeport setting isn't a particularly high-level one. In the campaign book the majority of NPCs, including the movers and shakers, are below 10th level with some notable exceptions. In my days of playing it the setting theme has fared largely around gritty urban adventures, sailing on the high seas and exploring tropical islands and archipelagos, and ancient ruins from forgotten eras.
High-level magic and heroism seen beyond 12th level just doesn't gel with Freeport and what I feel is right for the genre it's trying to emulate. The highest-level published adventure, Black Sails Over Freeport, ran from 6th to 14th level, but the majority stuck around 1st through 9th.
So in line with my imagining of Freeport's tone, I could comfortably do 15 or 20 point buy (10 is way too low for Pathfinder in my opinion unless you're doing some unorthodox 'peasant underdog' thing). Magic can still be common (they have a temple district and mage academy, after all), but the majority of what you'll see is probably fledgling adepts and the like. The Gauntlet of Spiragos free adventure by Nocturnal & Onyx Path has a variant adept for covering arcane and druidic magic, which you might find interesting.
As your campaign is dealing with cults, you should try sprinkling magic items of the evil-ish and macabre variety here and there. A few might be cursed, a wondrous item blindfold worn by a vampire might grant deathsight, stuff like that.
Freeport is essentially Call of Cthulhu in a D&D pirate city. Lower point buy and power level works fine for that but so can normal D&D/Pathfinder power levels. Power level is sometimes irrelevant, investigations are not based on CR and it is easy to make cultists and mythoseque critters powerful. Pathfinder has a ton of mythos appropriate monsters in the bestiaries throughout the power scales. I remember reflavoring some ooze from the Bestiary 2 as something cultists had summoned when I was running a Pathfinder Freeport game.
Funny you should ask... I've been running a Freeport in the Shackles PbP since 2011 >here<
We went with 20 point buy, but 15 should be OK. Freeport is grittier so lower powered is fine. My players are nearing 6th level now and I'm pondering going E6 or putting them on an even slower track of advancement just to keep the good times going without the power levels getting too high.
I think Freeport benefits from lower power and magic to reinforce the dark and dire tone. We've also had a good deal of fun with the insanity system.
The campaign info tab has info on my take on insanity, brief adventure recap and published adventures used, should it be of use.
Assuming you are comfortable with 15 point builds in other settings, it certainly should work in Freeport!