The 3 Series Coupe (E92) has more rear interior space (width, legroom, and Headroom) than the 6 Series Coupe E63 or even the actual F12.
The E92 is a lot bigger a lot more useful. The E92 sits 4 passengers much better that the F12 with it is a 2+2.
With the new "4 Series Coupe" (F82) that sleek rear roofline makes me think this car is gonna have less rear space that either of the 6 Series or the actual E92!!!
Even if there is enough headroom (which i doubt) the rear passengers heads will be just below the read glass (look at the pics). That heads are gonna get really hot on sunny days. Heads must be below the roof, not below a glass!!
I don't know why new BMWs are longer and wider but interior space (mainly headroom) is smaller and smaller.