Every teacher implements the program slightly different so your school might approach things differently. Even in our school they don't have the volunteers start until mid-Oct or early-Nov so they can get the classes into a routine before kids start getting pulled out.
There is definitely room for differentiation but as I said before it is highly teacher related. Some teachers are fantastic at it, others not so much. Some teachers are also mostly focused on the bottom of the class especially when the top ones are well behaved and coasting along so it might take some advocacy to give her some more challenging work. Other teachers are fascinated to dig deeper and challenge the top.
There are also some things that are easier than others to differentiate. DS does wonderful in science which is much more open ended and he can go more in depth but we still don't have a solution for math class since he isn't the type to do all of the grade 2 work and then sit and teach himself more advanced math with extra worksheets so for now it is a work in progress.
If you do have a chance to volunteer in the class I'd highly recommend it. It has been invaluable in seeing exactly what is going on and getting beyond the "nothing" answer to "what did you do/learn at school today?". It has also helped me better understand what is and isn't possible for them to do. Last year DD's class had 29 kids which included 3 special needs and 3 ESL. Some days I was amazed the teachers were still standing at the end of the day never mind that any learning occurred

Lastly, it has made for a better teacher/parent relationship and greatly increased communication. We are far from a perfect school situation but at least we're slowing moving in the right direction.
Good luck!