DD has worked her way through Walsh papers 1 -10, getting 70-80% for the first two and then high/mid 80's % for the middle ones, and now has dropped to 84% for 9 and 10. It's my feeling that the vocabulary at least, gets harder as you work through the papers. Her average score hasn't really changed and I think that's because the papers are getting more difficult even as she's getting better. It is always vocab that is her downfall with these
- the maths is definitely harder than other papers but don't think it gets progressively more difficult. The codes don't seem to cause her a problem.
We have found them very useful for practising 'moving on when you don't know the answer,' and although her scores haven't really improved, her timing certainly has.
Watch out for paper 12 as it has at least one completely different question type!
I would hope that if I set DD paper one again now, she would score 90% so I think that the answer to you question is yes - they do get harder.
Sorry for rambling - it's well past my bedtime!