Why did you decide to use ferrite magnets vs neo's?
They're a lot cheaper. Less than $1 for a big 2 x 2 x 1" thick magnet vs $8 for a 2 x 1 x .5" thick neo magnet. The neo magnets were double that price when I built the first ferrite generator. They have come down in price some now.
Would a 2.5 times more powerful neo magnet allow the elimination of the 1:2.5 transmission?
Yes. Although I have also used the transmissions on neo magnet turbines as well. Using the gearing allows you to develop the same power with a smaller and lighter (and cheaper) 12 pole generator instead of using a 16 pole on a 3.8 meter turbine, as an example. So it still cuts the cost of the magnets because you don't have to use as many of them.
Did having the transmission make anything easier or better?
The electrical efficiency of the generator. Since I started building geared turbines about three years ago now, I have not been able to build a direct drive one that can match the efficiency of the geared ones. Especially with axials - the bigger you make an axial the bigger around it gets and the more copper it has in it. The more copper you put in it, the higher the internal resistance. You can even double the copper up (two-in-hand windings) and can't match the geared ones with a single winding in delta.
Hugh (Piggot) built a new design ferrite magnet direct drive turbine in the 3 meter class using my "pole cramming" method to keep diameter and weight smaller, after he saw that it works. His turbine is pretty respectable from homebrew standards as compared to contemporary neo designs and it puts out about 750 watts. But not even close to the 2.5 kW I get out of my geared ones.
How much of an issue is corrosion with neo's?
Pretty bad in some cases. Ferrites can rust, but they never bubble up and form white balls of corrosion like neos do. The only thing that protects neos from the weather is the coating on them. Sealing them in resins doesn't work because moisture trapped in the resins during casting gets in, destroys the coating, and they bubble up and fall off. Neo magnets are very, very fragile.