I think that is something that will have to be evaluated in the flesh. Looking at the measurements I find it hard to believe that the CLA will actually have more leg room. But that is exactly what the chart says. However, I've seen enough cases where the charts did not translate to actual room in person.Cars like 3er sedan, C sedan, A4 sedan etc are FAMILY SEDANS (at least in Europe)... and in vast numbers bought by such clientèle ... meaning they care about rear headroom & rear space (space across the rear bench). Both are much smaller (~5cm) which is quite a lot - especially in the headroom case. I don't see much cross-shopping. It's like saying CLS would steal 7er customers. Not really ... at least not much.
So, if the CLA has the rear legroom it shows, then I could see several buyers opting for it instead of the C. Especially the younger buyers.