Shouldn't the most immediate response be, "why aren't you memoizing"? ;-)

So I think the idea is that users be able to suggest test cases breaking some existing solutions and probably get rewarded for it.
As for suggesting anything, you can write that in a comment; without a reward, though. As for "Playing with digits", the kata, like a lot of others and unlike competitive programming problems, is not defined strictly enough. It isn't clear what the intended difficulty was meant to be and thus whether big number tests should be there.

You can fork any solution and add tests, change code, etc. Is that what you were looking for?