Today I pulled the mousse out of an Varg EX rear wheel with the Michelin 140/80-18 Medium. Now the Michelin got a lot of volume, usually more than the Metzeler, the mousse has a cross section circumference of ~310mm and says "made in Austria" on it so I assume it is an X-Grip EH2 - seems too small to be an EH3.
Now if you look at the Nitro mousse sizing table
Fitment Chart Search, the same 18-325 is shown both for the Metzler and Michelin that can be chosen stock for the Varg. Nevertheless for the JXE 140/80-18 the smaller 18-305 mousse is recommended. What does this mean? Most likely your mousse will be a tad too big for the JXE, but you can always cut out a piece and save for later when the mousse is worn. Will it be a perfect fit - no, but with mousse you never know until you try!
Michael