I think there is some misunderstanding about heat and speed here.
The reason for heat is that the current density is being pushed too high for the size of wire. People are trying to get too much power for the size with the difficult loading into batteries at constant voltage. You push the alternator efficiency too low and more power is lost in the stator than goes into the battery.
With high speeds you can use less turns of thicker wire so the losses are smaller. With a prime mover other than wind ( engine, hydro etc) you only need to run it fast enough to provide the excess voltage over the battery to drive the current you want, the efficiency remains high and stator heating is a far smaller issue.
Any low speed alternator needs to use lots of magnet and copper to remain efficient. The matching to a wind turbine actually forces us to keep the electrical efficiency low to keep the blade speed up, so it is cost effective to place this low efficiency in the alternator. If you make a big and expensive alternator you will need to add loss in the line for the same power out of the blades. This is expensive so you try to compromise with an efficiency that matches the load and keeps stator heat within reason. Far better to use a more powerful alternator and keep the losses outside but it comes at a high initial cost.
This still applies to iron cored machines, but they dissipate the heat better and can be more severely overloaded without exceeding the wire temperature limits. if you match the air gap machines to the load then this heat problem doesn't get out of hand. Incorrect loading of a motor conversion may not burn it out, but the low efficiency still reflects in the power out.
Perhaps a comment I made earlier about keeping iron in the centre disc of a 2 stator machine may not be out of place. Magnetically I suggested that iron is not necessary but mechanically you need to remember that these discs ( all of them ) are fairly well stressed axially by the magnetic attraction forces and the limit to diameter of axial machines is probably decided by the strength of the discs and their stability against the magnetic attraction.
In theory the centre disc is balanced in pull, but the condition is totally unstable and you can't manage to keep this force balanced. There is a big problem during assembly but it never goes away completely. Any composite material will need to be completely rigid axially as well as holding the cf forces on the magnets. If you can satisfy this in proximity with a fairly hot stator then ok.
You can mount the magnets in holes in stainless steel discs, but I fail to see the advantage when they can be stuck on to mild steel that provides mechanical strength and carries the flux. Using 2 magnets on the centre disc gives more flux than the single magnet in a composite or stainless disc.
For wind I strongly advise against stacking stators, it is not cost effective, has worse cooling and gives a lower output for volume of copper and magnet.
If you must produce engine driven PMAs ( or for faster hydro) then stacking may be sensible but don't ignore the mechanical challenges.
Flux