Measuring the productivity of dual purpose vs diary cows and temperate vs tropical climate

A friend that is going to Fiji and is looking at owning a goat or cow or milking made the comment that cows in cool climates can give 24L of milk where in hot countries they can only give 2L.  I presume this is meant to be based on per milking and twice a day milking.

I raised the issue that the cows that can give 24L of milk are also fed a lot more food to get the 24L of milk.

This made me thinking - how do the two compare when these are considered...

- How much feed is required per litre of milk in a 50 and 4L per day cow?

- IF you also value the calf and plan to breed them for meat, how does this affect the equation?  (E.g. What is the total food value of 1 dairy cow and one calf, or 3 dual purpose cows and 3 calves and what is the feed required to produce them).

- Could 50L per day be obtained from a fully grass-grazed cow or do intensive supplements need to be given as well?  If they do, what is the environmental cost of the extra milk?

- Does temperature really affect milk production that much?