Your understanding is correct, and the math is pretty simple. Swords Into PlowShares (SIPS) is 15% bonus to excess food. Feed the world is +2 food, but only after you build both shrine and temple (so delayed on a new city usually). The benefit of it being flat food is it will catch the natural bonuses of ToA and Fertility Rites and Floating Gardens. (Remember Aztecs already have the equivalent of SIPS with floating garden and extra food on lakes) So that 2 food becomes 2.35 or so in your case. So if you keep an excess of larger than 15 food at most times, even early game, then SIPS will exceed the benefits of feed the world by adding 2.25+ food to that amount. A good way to make sure you're getting the full effect of SIPS is to feed your capital with 2 food ships. This almost always causes a large excess of 15+ even early game which means SIPS will be better most of the game, growing your capital larger.
However, keep in mind you need to be at peace. A lot of ppl forget that fact and don't factor it in. The ability will deactivate the moment you get involved in anything which is kinda annoying. As this usually happens a bit even on the most peaceful game, I'd want to keep a bit more than 15 excess food to compensate if I intended to use it. SIPS is also magnified by putting the city on food focus and is nerfed by switching to production focus so it ends up giving you pretty unstable growth. So this is the weakness of the belief. Feed the World is the same no matter what focus, etc. You may see a big turn difference in growth just by switching a few tiles around so I'd definitely want to micromanage to get the most out of it.
So Swords into plowshares could be better and usually is better later in the game when there is a lot of excess food. Ever checked the surplus on your 40-pop cities? It's usually pretty huge: at least 30+ extra if you still have decent growth. So for that, swords into plowshares would yield 4.5 food, more than double feed the world. Therefore, if you intend to play up till 2050 it will probably end up better for you for reaching a higher end-game population. Just keep your cities on growth emphasis most of the game and don't take them off.
I would not count on it being of much use mid-game though. Like Fallout said, with all your other growth bonuses you are going to be growing fast and are likely to run into bad happiness problems which will waste your food buffs by 90%. Once this happens SIPS is made less than useless. I tried the Aztecs for speed growth and could not keep them growing midgame--happiness ran out too fast. So you may run into the problem where you have a lot of potential but can't keep it going because your bonuses are too high. Definitely score patronage and try to get all the city states if you intend to play this way. If you want a capital size record then I'd recommend totally nerfing your extra cities. Maybe even stopping their growth at size 5 or so (just enough to work university slots and not starve). It is far easier to grow the capital as it gets 1/2 happiness so then you could get around the problem of the midgame happiness shortfall. You could just use your other cities to send a food ship each and build little things like military. Speaking of which, you will also grow far better if you have a bit of coast as food ships are double the food of caravans. You'll get +8 food per route.
Perfect answer, thanks
I played the planned game last night and it was actually quite crazy
I got a coastal start with 5 lakes, 4 citrus and 1 banana. I snagged Sun God, Built Temple of Artemis AND Stonehenge (risky to attempt on Immortal but I like pursuing wonders early rather than the boring rush to NC). Once I landed Swords into Plowshares my capital growth was just ridiculous.
I made the error of wiping out Siam early however. Theodora annoyingly settled 5 tiles from my capital and took one of my Citrus, hence I needed to forward expand near Siam instead, angering him and starting war. He somehow only had ONE city, which is quite weird as I have the aggressive and expansionist AI Mod running. After I took it I copped an extreme warmonger penalty and everyone took successive turns DOWing me. I wouldn't have taken Sukothai had it not been in a PERFECT position to send food ships to my capital. Theodora's forward settling of me had gotten rid of any other viable spots for that.
Even with STPS disabled, my capital reached size 30 by the Industrial Era and I'm now #1 in science. Hopefully when Ideologies come around other Civs will stop hating me so much.
# On a further odd note, Theodora constructed Petra in a 1 POPULATION city using a GE, which I captured when she declared war on me for the 2nd time. The city also has Wine and Copper and was quite easy to take with X-Bows.
Overall, I should've probably opted for a more peaceful route, but I have gained a lot from enemy cities despite incurring the wrath of other Civs.
Oh, and to further add, my happiness problems have been non existent aside from unhappiness incurred early from newly captured cities. I've went Patronage and allied a fair few CS, including a mercantile one. I also rushed Forbidden Palace with the GE from my early wonders. I somehow also hard built Sistine Chapel, which Ive never done before.
Post pics?
