It's not THAT inefficient using getting warehouses for food, in fact in situations where it is not viable to build housing near the food source, in my opinion it is often the best option. I've used it to feed fish to 19,000 patricians in a contest (Palm Springs), and also found it very useful in Lugdunum career mission.
Suppose you have one getting warehouse, whose cartpusher has a 240 tile round trip to the warehouse he gets from. This takes just on four months. If the getting warehouse is very close to the granary, he should be able to deliver 4 loads in another month. Thus the output of one getting warehouse would approximate that of one farm (not wheat in central or desert climates), at an additional cost of 6 workers. It can be higher if the distance between warehouses is less.
There are caveats to using this strategy. It is highly recommended that all food initially be sent to just a single warehouse, which may itself introduce inefficiency. Otherwise the getting cartpusher will select a warehouse that may be empty by the time he arrives (as you'd be running several to many getting warehouses, not just one). This can also happen with a single warehouse but the idea is to balance the supply with the demand.
The other issue is that it can take many months before you first see food in your granary, as the getting warehouse has to have 5 or more units before it will attempt to fill a granary (so long as it is set to getting). This can be overcome somewhat by initially using multiple carpushers, if you don't consider this to be cheating, or by manipulating warehouse orders.
Warehouses count as 9 buildings, so getting warehouses are not the best if you are likely to exceed the building limit.
[This message has been edited by goonsquad (edited 04-16-2011 @ 08:49 AM).]