The improvement depends on how the alternator behaves on load. you only have open circuit volts to work from. As soon as you take any load the open circuit volts fall due to the internal impedance. Without a load curve of your alternator you can't predict what will happen.
You are dealing with very low power so I feel certain that any transformer unless carefully chosen ( and very expensive) will actually make things worse.
The buck converter alone will do the voltage transformation better than using a transformer. As I said previously there should be an improvement but it may not be great as the alternator is basically not stiff enough to absorb much additional power from your blades. For good results with a mppt converter you need a stiff and efficient alternator that if used directly without the converter will bring the blades hard into stall just above cut in speed. The real factor is that your alternator can give lots of volts but to get significant power it needs to be driven very fast even when matched correctly.
If you want to get some idea of the improvement possible try charging a 24 or 36v battery. The high wind result will be similar to the converter on 12v. You will loose the low wind power with the high voltage battery but that should come back again with your converter.
If you get big improvements in high wind with the extra batteries then it is worth trying the converter. Rember to compare wattage not amps with the additional batteries as you don't have the converter changing the current ratio.
I am convinced the 540 is far too slow for the size of blades it will support, it must be designed for some very slow blades just to get a few watts in lower winds. The slow blades and reactance limited windings probably let it survive in high winds but with miserable performance.
To get anything out of a car type alternator you have to do as in the "Air machines, ignore winds below 12mph , run furiously fast blades with cut in well over 500rpm and do something to protect it in a gale. When you do that you get useful power in high winds and for marine use it seems to be fairly good. For land use it only works on the best wind sites.
Normally 130rpm cut in would more suit a 12ft prop. Your little thing won't fully load a decent 4ft prop in high winds, the low cut in speed is pointless and detrimental. You may well do better to open the star point and Jerry connect it but not the series connection that Jerry showed, you need the 3 bridges in parallel as your winding is far slower than the standard delco alternator.
