So when you sit down across from Kate you know she has 15 influence available to her.

When you see out of faction cards hit the table you can make inferences about what threats they could possibly have left.

How does this change with MWL? Are players required to inform their opponents about their influence maximum? Or is it all hidden?

Well, the influence is still 15. It's just less for the inclusion of MWL cards. When you see a Clone Chip hit the table, you account for that the same way you account for seeing a Datasucker, and keep the running total of 'influence spent' in your head.

The deck should still be legal, but there's no reason you should know any more about the exact deck makeup than usual before the game begins. If they tell you they have 11 influence instead of 15, you know to expect certain cards, which goes against the intent of the game design.

It's at least an interesting question. However, I have to agree with Feesh.