Just a little reply to my own post. I just tested with the 1.09 patch whether researching prereqs first is quicker. I had one city, of population one, using a tile grassland forest tile whilst building a worker (so no extra commerce and no growth).
Agriculture has no prereqs, Animal Husbantry has prereqs of Hunting OR Agriculture. The germans have hunting, so I can research Animal Husbantry straight away. So I can research these techs in either order.
Note that Animal Husbantry is 140 beakers, Agriculture is 80 beakers.
(1) Animal Husbantry: 12 turns @ 12 beakers per turn, (4 overflow)
then
Agriculture: 8 turns @ 10 beakers per turn. (0 overflow)
(2) Agriculture: 9 turns @ 10 beakers per turn, (6 overflow)
then
Animal Husbantry: 10 turns @ 14 beakers per turn, (6 overflow).
So researching animal husbantry first resulted it a turn saving plus a extra 6 beaker overflow. I'm not sure if was the result of me researching the prereqs, or another civilization discovering the tech and hence reducing the tech cost for me, but I think it was due to me researching the prereq, Agriculture.
I believe this is true because the additional beakers added each turn to Animal Husbantry was always 14. Which means if another civs tech was affecting my tech cost, that civ would of had to discover Animal Husbanty before I discovered Agriculture, which I think is unlikely.
Note that I can only find my palace producing 8 commerce, so I don't know where these extra beakers are coming from, and whether they're from an additive or multiplicative effect.
Now, admitedly, the effect is only one turn and 6 beakers, so depending on your resources and whether you want to find where the horses are might make it wiser to go for Animal Husbantry first.
However, consider this situation.
Tech B costs has optional prereq tech A.
Tech B costs 10 times more than tech A.
This situation occurs a lot later on.
I'm going to suggest that it may be in some circumstances quicker to research A then B, than to just research B. Not only do you get B quicker, but you get a free tech. This will need some testing, I'm not sure if the discount for researching optional prereqs is that great.
