What tyres do we have to buy to get a free (real) magnesium wheel, or are
 you just saying that buying a new lighter tyre will effectively give us the
 same performance benefit as buying a magnesium wheel instead? 
 why not get both a new tyre (light weight), and also a set of carbon rimmed
 wheels to go with them?  then, braking, quickness of cornering /
 flickability, and accelerating will all be much improved.
 
 In addition to the inertial benefits of reducing tyre and wheel values,
 there are additional improvements that can be made to the bike  (works on
 cars too) of reducing the unsprung mass, that is the mass that is on the
 road side of the suspension.
 
 This mass, if reduced, will also gain significant benefits for the
 performance and handling (primarily handling) of the vehicle, as there will
 be less mass that is not compensated for by the suspension.  this will help
 in holding the desired line through corners, with less slipping and sliding,
 and less deflection off the chosen line due to bumps and deficiencies in the
 road surface, it will also mean that there is less mass that is being
 allowed to affect the vehicles handling, as the suspension works very hard
 to not let the bikes mass affect these aspects.  this is also why a stiff
 setting on your suspension will actually have a detrimental effect on
 handling, if it is set too stiff for your bike, the rider, and the types of
 road the bike is being used on, as it is not able to absorb the bumps, so
 gets thrown off the 'perfect' line.