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.