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Excellent thread and really useful to me as I'm currently sorting out an 'alternator optional' system on my 2002 VW Golf TDi (PD).
I did some measuring and found that I gain a bit less than half an mpg for each amp of power I can persuade the alternator not to produce.
Since I will be plugging the car into the mains every night to charge the battery, I'm also going to add a mains-powered engine pre-heater system - to be used year round - so that the car starts each day at normal operating temperature. That ought to save quite a bit more fuel as my engine is always much less efficient for the first 20 minutes or so, even in summer.
If you haven't already done so I can really recommend actually measuring the electrical load caused by running various equipment. I was surprised at what I found. I had no idea how much power the blower used, even at low speed, ...and I was equally surprised at how little power was used to keep the engine running. Simply not using the heater blower fan nets me an extra 3mpg on its own. Conversely, the radio uses very little power. I haven't yet started running with the alternator disconnected, and of course when I do these power-saving tricks won't save any fuel exactly, but they may extend my alternator-free driving range to the point where I can get away with using just one battery rather than two, and avoid having to use an expensive DC/DC converter between the two batteries.
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