Paul,
I agree with you figures if using 2mm copper wire, but typically you would use 10mm or 16mm cable for battery connections.
For 2mm copper cable the standard resistance per meter is 0.005488 ohms.
For 10mm copper cable the resistance per meter will be 0.0002195 ohms. (by my calculation)
For 16mm copper cable the resistance per meter will be 0.0000857 ohms.
(resistance is inversly proportional to the square of the diameter). In other words, the thicker the wire the less the resistance.
So, for 5.5m of 10mm cable the resistance will be 0.0002195 x 5.5m = 0.0012 ohms.
Which gives a voltage drop of 0.0012 x100A = 0.12 volts (for 10mm cable)
For 5.5m of 16mm cable the resistance will be 0.0000857 x 5.5m = 0.00047 ohms.
Which gives a voltage drop of 0.00047 x100A = 0.05 volts (for 16mm cable).
By my calculations, it shouldn't be a problem fitting the battery in the console, if you are using suitably sized cables, so I have to agree with Whippet. Just make sure all the connections are tight.
Also, a Marine Battery is designed to be able to start motors as well as running your instruments. A Leisure Battery is designed primarily for running your instruments, and not for repeated starting of a motor. So, if you fit a Leisure Battery as you second battery, try to only use it for occasional starting of your motor. It will start the motor, but you will eventually damage it if you keep trying to start a motor that is relunctant to fire, for example.
But with all the "senior moments" I've been having recently, it might be an idea to check my calculations.