Sounds right to me more coffee.
However there is probably a cap on the input of the recorder, it wouldnt be bad practice to include one though.
And with more load on the electrical system there probably will be more ripple making the recording 'louder', if you plug 'phones into your recorder, start the engine then switch on headlights, brake light etc you'll hear the changes.
I'm not sure I'd want to rely on performance data provided by this method to go and change ign timing etc, you'd have to use the same piece of road in the same conditions etc to get meaningfull results.