Agreed, the increase in diameter is minimal and the Badlands perform better on all surfaces than the standard tyre as well as looking better, the standard one looks a little smaller in comparison with the front IMO.
My first M5 was secondhand and came with a Badland fitted over the standard tyre (which had been worn almost bald) and spot glued in place, I wore that one down, replaced it but used glue all round this time to try and avoid getting moisture beetween the two tyres.
It worked very well, even allowing removal/reffiting to service the rear wheel/gyro etc, however when the second badland wore down it was difficult to remove the badland without damaging the standard tyre (but no moisture
So I bought a standard tyre to wear down and recap, I'm still undecided what to try next but most 1/8th buggy tyres should fit. Loads of choice and cost effective too