its a new sensor but probably build with the same design as all other APS , 24x36mm sensors from Canon since 2004
We can't be sure.
At the CMOS/electronics level, the 70D sensor is a 40mp sensor, basically.
That seems way too many for Canon's old 500nm process - when even 18mp was pushing it.
I guess we'll find out when Chipworks puts the 70D sensor under the microscope.
Might the pixels being split for AF hurt ISO performance by a stop?
The Mark III (and I'm assuming 1DX even more so) has good high ISO performance. It's low ISO that suffers. If anything Canon would want to fix that but I don't think they'd want to publicize it since the problem is not really well-documented except in niche communities. (Noisy ADC or somesuch.)