I guess the question is how much more, but 1.5mm? Really? That's a difference of around 0.2% on the rolling diameter. I find it very difficult to believe that would cause a problem. Tyre pressures and temperatures will make a bigger difference than that.
I think it's bad practice to put tyres with significantly different wear on the same axle purely from a performance point of view and if I destroy one tyre I will generally replace two, even if it's with the spare. Of course dealing with a flat tyre on the road is completely different.
Cheers,
Jon

