Which brings me to my main question for you:
Surely a good 20k requires a coastal city? Otherwise you lose Colossus, Lighthouse and Magellan's. Colossus is going to be important for getting to Republic quicker (and all the other AA Wonder techs). Lighthouse and Magellan's might be passable, but they are cheap and the Lighthouse can be made very quickly.
So, if you're going to need a Coastal start, it's going to be very difficult to also have 20, or even 18 productive squares is it not? That one start location that includes both a small Coastal start and a 30/35 production potential from 12 squares, on a River, with bonus foods to feed the many Hills, is all going to be quite a tough one to reload for? So we're back to point 1 really aren't we? It's 90% all in the start location... ?
I'm not sure. I've mainly been playing GOTMs for the last 7 years or so, and there we can't choose our start position... (Of course, accordingly the 20K finish dates in GOTM games are usually much slower than in the HoF games.) If I remember correctly, the last couple of times that I went for a 20K victory, we had inland starts. And here ST really makes a difference. 21 tiles instead of 12 is, well, not quite double, but it's
a lot.
So the question is, if you can actually
choose your start position (as in a HoF game), would you then choose a coastal start or an inland start? Lighthouse and Magellan can probably be replaced by other wonders, doesn't hurt much, if these are the ones the AI gets. But not being able to get the Colossus really hurts. It's a great wonder and it can start accumulating culture from very early on.
You may be able to compensate for not being on the coast by having access to ivory, because SoZ is also a great culture wonder. But of course, having both, SoZ and Colossus, is even better...
So if I could choose my start position, I would probably take one with access to ocean in such a way that only 2 tiles are required, leaving 19 tiles for production, and of course ivory close by and iron+coal in the BFC (so that I can build the Iron Works...)
And yes: the finish date in a 20K game is determined to 90% by the start position. The remaining 10% are luck factors like when which AI decides to start which wonder, how lucky you are in the wonder cascades, or when you get your first MGL (so that the Heroic Epic can be built). The HE is also a great culture building. 4cpt at only 200 shields, and if you can get it really early, you don't need to wait so many turns before it makes 8cpt... (Also the SGLs are a luck factor, but as GOTMs are always played with SGLs disabled, I haven't spent much thought on them so far.)