The total depth of tread rubber on a racing tire tends to only be 2-3, maybe 4mm. Wear will be somewhere less than that.
Lot of things are going on as a tire is raced. Wear absolutely will affect both loaded and effective radii. If inflation pressure changes as tire temperature goes up or down, that will also affect things. Likewise, you can have carcass degradation as well.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.