I first posted this question in the cyclocross forum, but I think that some tourers might have tried this.
Shimano states that the maximum cassette size is a 27 tooth for an RD6600 and a 28 tooth for the RD6700.
Will a 32T MTB gear work?
The reason I ask is a gentleman, 72 y.o., is using my old cross bike in the Paris - Ancaster race. He asked if I had a larger rear cassette to make it easier on him. For fun, I took a 32T cassette off my 29er and installed it on the Tiagra equiped bike and in works superbly. No issues at all.
Before I spend money on a new cassette, I was hoping someone already tried something like this and could offer some guidance.