You can use pull to compare identical shape magnets of the same size but otherwise it is no a useful indicator. It would take too long to explain why. I assume the makers or suppliers give this figure for those who want to use magnetic clamps and fastenings.
Unless you have a decent grasp of magnetics and electrics it's a bit difficult explaining all this but I will try.
For comparison of magnets there are a few factors that are useful. What you need for an alternator is the flux per pole crossing the air gap. The Br that you mention is the flux density that the magnet will maintain when magnetised in a closed loop and the magnetising field is removed. You can never get that value in an air gap, but it is a useful comparison figure. The N number is the energy product of the neo material and is the product of remenance and coercivity ( the ability to resist demagnetising).
The number is in megagauss Oersted but just the N number is the comparison. With neo the coercivity doesn't change much between the grades so the Br value is more or less proportional to the grade.As you show in your figures the Br increases roughly as the grade number.
Now the flux per pole depends on the magnet area, flux is B x area. That means that increasing magnet area results in more flux. The flux also depends on the air gap. If we use a tiny gap we have lots of flux and if the gap is large the flux falls. To obtain the same flux in a larger gap we need a thicker magnet and the value of B in the gap depends on the ratio of the gap length to magnet length(in this case we mean thickness by magnetic length)
If we build an iron cored machine we can use small air gaps and the magnet thickness can be smaller than for air gap type alternators. The snag with iron is that it saturates and for higher flux the thing eventually behaves more like an air gap so you need thicker than you would expect even with an iron core if you want much power.
Considering air gap machines you can get a lot of flux in a small air gap with thin magnets but unfortunately you can't get much copper in a small gap. You can get any volts you want with thin wire but the resistance will restrict the current and hence power out. The only to get more power is to use a wider gap to take thicker wire and that means using thick magnets.
Even with neo magnets You need a lot of it for much power, you need very much more with the lesser grades of magnet such as ferrite and magnet steels.
Using the highest grade neo cuts the volume required so you would save about 12% by using N50 over N40 but unless size or weight is the only factor you may find it cheaper to get the same result with more N40. Also depends on where you buy it and the deal you get but in general anything over N40/42 is not cost effective.
Things don't end there, you also need to consider the wind related bit. The alternator needs to match the prop. If you do something to make the alternator more powerful then you mess up the match to the prop and to keep things on track you need a bigger prop.
You end up playing with many variables, magnets, turns, wire size etc. You have to have an alternator with thick enough wire not to cook but you need to match it to the prop if it is to work well. Sometimes changing to a higher grade magnet in an existing design may bring no benefit and increase the cost so it's not easy.
If you take all this into account at the design stage you may be able to produce a better machine with higher grade magnets but it may end up needing a bigger prop or a wider air gap for the same prop and the cost may be higher for no improvement if you go that way.
Hope this helps, I know its not easy.
Flux
