I think I have to setup a model to compare SP and Corporation, since GE is really a problem if there is no stone around.
Assumption 1: Communism and Corporation can be achieved at the same timeframe. (Actually Communism can be obtained earlier.)
Assumption 2: 20 turns are needed to spread the corporation to all cities. (20 can be a variant)
If you have AL before corporations, then less than 15 turns is usually enough to spread corporations to an empire of 40+ cities, this was how we did in our SGOTM12.
Assumption 3: the upper limit of city# are 60 for SP + no vassal. With corporation, only 30 cities can be built because of the colony expense.
The major factor that limit the number of cities is the domination limit, it also depends on how many islands and how many continents in this map. For instance, if there are 2 continents, then the land tiles in our own continent are almost close to the domination limit, you can take at most ~10 cities from other continent. The colonial cost of 5 cities are trivial, even at 10 cities, the gain from corporations easily surpass. Check this http://forums.civfanatics.com/showpost.php?p=10049967&postcount=5, I had average of more than 10 cities in 3 other different continents and could still produce 13.6K bpt without deficit.
Assumption 4: From the communism to FT1, 60 turns are needed. (60 can be a variant)
With SP, the city number can be increased to 60 with a snowball effect. If the additional 30 cities are conquered in 20 turns after communism, most of them can contribute at least 30 turns. Supposing each city can contribute 60 bpt (food/hammer can be converted to beaker), the additional 30 cities can contribute 54K by the end of game.
Comparatively, the SP effect on the first 30 cities is only 525 bpt/turn compared with FM.(estimated as 4 food+ 5 commerce+ 1 hammer) but it will take effects for 60 turns. It will contribute 31K.
With Corporation, I'll use Duckweed's result. As the executives will cost 9K hammer, the 3 corporation has to reach 31K + 54K+ 9K=94K in 40 turns, which is equivalent to 78 bpt/turn*city.
Therefore, we can find that the advantage of SP is mainly determined by the snowball effects on the city numbers. In this model, if the bpt of 3 corporation can reach 100 bpt/turn*city on average, corporation can shorten the game length by 9 turns.
However, if the GE is kept until turn 200, will it also delay the victory by 9 turns? Probably not. For Louis, let's get a forge as early as possible.