If my math is right then we have 2*(51!) ~ 3*10^66 combinations. That's quite a big number.
And for each of those combinations the key has to be converter to address and checked.
Maybe somebody can further (greatly) reduce the numbers, but I don't know how.
I believe that people are not going to waste their time for 0.002B. If someone has a system that reduces the combinations (I doubt it) then maybe they will try it but it is a lot of effort for so little reward.