I think the main problem is that we are looking at such minor differences here that any performance gain or loss can be masked by other factors, whether it be variation in driving, track temperature, air temperature, wind, and the length of the race/session. Driver error being less in the higher classes, where changing rims seems to make more of a performance difference.
While it is true that Mg and Al have different mechanical and thermal properties (and different densities); without knowing the actual dimensional differences between the wheels you can't say which will dissipate heat quicker/be stiffer/perform better. Simply saying 'Mag is better than Ali' is rather general.
However if you had a specific set of each, you may be able to say 'THESE Mg alloy rims perform better than these Al alloy ones' or vice versa in the wet, whatever the logic. I think most of the time Mg wheels are lighter than Al wheels, but I don't know how much lighter, or how much weaker or softer they are.
You might get one set of rims that heat the tyres up quicker, but the other set may retain heat better than the first. Of course, quickly obtaining heat is always wanted up to the 'right' level (anybody care to divulge what the optimum tyre temperature is? Can imagine it changes depending on the tyre construction), but retaining heat is needed OR unwanted in different conditions.
It probably is right that Al wheels perform better in the wet than Mg, but nobody ever seems to know by HOW MUCH.
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