dc42:
If you increase the strength of the magnets, you will find that the no-load speed of the motor decreases for a given supply voltage, but the torque increases for a given current. You could increase the voltage to get the original speed back, but if you do that then you should current-limit the motor driver so that you do not feed it more current than it would have if it were running from 6V.
Absolutely - from simple physics of a conductor moving in a magnetic field, V = B x L x v (magnetic flux density x "length" x velocity).
Increase B and v goes down.
However the value of B to use depends on the magnetic circuit - with ferrite permanent magnets the flux density is likely to be about 0.4T in
the magnets, but could be a lot more in the stator pole pieces because of their shape (perhaps 1T or more). The maximum flux density in iron alloys is about 2T (neodymium magnets are about 1.4T and are likely to saturate the iron). I'd expect an increase in torque over ferrite magnets of a factor between 1.5 and 3 or so.
Actually the air-gaps can be a limiting factor to the flux density too - so the whole thing is rather complex and it would be interesting to
see what difference you get.