However, one shift, particularly in Maths, is for depth of understanding and problem solving rather than acceleration onto 'different content'
I am not sure how much I buy this argument. I can understand if you move higher up (towards the end of secondary school, say), but how much more in depth can you go at primary level? I mean, there are only so many "shirts that you can buy at £24 each to sell them at £30", or there are only some many "sheets that the teacher can give equally to the class till she has run out of paper", to give you just a couple of examples.
Besides, the corollary is that it is assumed that a "normal" (if you pardon the discriminatory and belittling label) child would end up without the same amount of "depth of understanding", which seems rather odd to me.
And, even if the "able" child was given tons of material to gain a better insight on whatever topic, the bottom line is still the same... the normal lessons would simply not apply to him, so I don't see what the real advantage, as far as class management is concerned, is.
In addition, I can scarcely believe that normally in a class you would find children working at +-2 years (or more) with respect to their theoretical level. I can understand that there could be children with learning difficulties, but that doesn't mean that there is always continuum from -2 to +2 years (or more). If anything, the teacher would go demented. :)