Just to reiterate, but as someone living in a country where we do not do "ability tables" for maths:
It leads to significantly higher maths attainment overall IME, but there are some downsides.
For one, parents are more likely to have to do extra work at home (also, you get more tutoring). The weaker kids will have to do extra practice of basics with parents to keep up to the baseline. The kids with a high level, meanwhile, will not have the same level of opportunity to do extension stuff at school, so parents will have to do this themselves (hence my suggestion to get workbooks from WH Smith and get cracking at home).
The other thing about whole-class teaching styles is that you will have to give up on the fantasy of 100% inclusion and accept that the bottom few-percent of children will have to have a special-needs stream and do their maths in a separate class. Some kids will not keep up with the class no matter how much extra practice they get.
It kind of sucks, but all in all I prefer it to the hitherto English system of putting "less able" (often simply summer-born kids, or from less middle-class families) into "lower ability tables," giving them easier work and letting them bump along the bottom, drifting further and further behind the more privileged kids, until they collapse into bottom sets at secondary school.
The important thing to remember that managing a spread of different attainment levels is a perennial and unsolvable problem in education. There is literally no way to do things which does not involve some trade-offs, downsides and negatives. Whole-class mastery teaching involves the downsides I describe here, yet overall I think it is the "least bad" option. The crucial thing, as a parent, is to know how to handle the downsides for your child.