Down 'ere it is done for economy .. so you can do the canning with 5 litres less fuel.
The original coil sits to the right front side of the main tube, behind that are the ignition black box, Voltage regulator, blinker relay, starter relay. Behind that you should be able to squish in a small coil. Putting a coil under the main tube .. well there is not that much space there and you make servicing that much more complex, you have to remove it to change the air filter for instance.
You do know about reducing the timing advance range?
Calling an 800 GS a dead beaver is like calling a guzzie a spaghetti bm.