External RAM with ARM Cortex M-3
If the microcontroller supports external memories then it will have dedicated pins for the address and data buses.  You can't assign just any pin to a bus.  I don't think you need to worry about using up all the pins.  Small microcontrollers with relatively low pin counts likely don't support external memories.  Read the microcontroller's datasheet.

Well I suppose you could say that there is always a bottleneck somewhere but the real question is whether the bottleneck poses a problem.  I believe the microcontroller designers would have thought about and minimized any bottlenecks when they designed the chip.  I don't think you need to worry about this. 

The serial interface is going to be orders of magnitude slower than the parallel interface.  You can't seriously be worried about a bottleneck on the parallel bus and consider a serial interface as an alternative.