Well, I just tired a normal/emperor/continents map with China. I decided to go heavy ICS from start. The land was rather poor (90% plains; some incense, gold, one dye). Since the land wasn't food-friendly I took a bit wider city-spread (3 tiles between) to grab as many riverside for each city that I could. Immediately after settling my 8th city I got a DoW from Rome (eastern neighbour) and a turn later from Japan (northern neighbour). Luckily I was beelining (and was 10 turns away from) Chu-ko-nu's and Rome had to walk 10 hexes or so to my lands.
All my cities had Paper maker by the time of DoW (or were finishing it). Rome showed up with 3 longswords and 2 ballistae. I had longswords and Archers. I lost one city but retook it with a newly spawned GG and some archers upgraded to Chu-ko-nus. Later I took all but one city from Japan.
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It's turn 175 I have 12 cities (3 puppets, one annexed for workboats).
Total population 55 (my highest pop city is size 7) including puppets.
Total science generated per turn: 86

(all cities have paper maker), 7 turns to NC).
Total income after maintenance 160

per turn (60 eaten by military, 38 by buildings, 27 by roads). At least 50 GPT is coming from outgoing resource trades.
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Conclusion:
I can see why ICS is appealing for Multiplayer.
1. Cities have only minimal infrastructure so loosing one doesn't hit you as much as loosing a big city.
2. Cities are easier to recapture due to lower population.
3. ICS can build a massive army in 15 or so turns, which in turn gets upgraded instantly due to massive GPT.
4. RAs and resource trades are virtually non-existent so GPT boosts happines via city-states and rush-buys of colosseums.
That said, 86 BPT at turn 175 is simply horrible. I know that the Colloseums I'm currently building will produce a population boom (10-size cities) and that this research will double in, say, 50 turns, but quite frankly, with my "regular" non-ICS playstyle I get even (200 BPT at turn 200) faster.
With a smaller empire (say, half the size) I get the following benefits:
- less workers needed (less upkeep, build time and rush-buys)
- NC goes up faster
- road costs are slightly higher per city but overall lower
- I can afford having a smaller army (less upkeep, build time and rush-buys)
- I can afford to sell more resources, sometimes even non-duplicates
- I can afford to build wonders
- I can afford to go 2 points into Tradition (captial + wonders) and are not forced into Liberty
- Golden Ages come more often and thus my production and GPT is even stronger.
- Social Policies come more often and I can afford going into Piety (to eliminate the need for happiness buildings past Colloseums, which I build @ T200+).
If I ever need to get more GPT I can always conquer a civ and puppet twice or three times the number of my core cities. As that's the only real benefit of ICS, I can say ICS is pretty obsolete. At least in single player.
EDIT: Attached screens