Very odd thread! Ultimately you should buy what you can afford to run. If saving £50 year by having a box plugged in is a big deal, when it costs £90+ just to fill the car up, I really don't think its a big saving at all, just don't put your foot down for a few weeks (like you cant with the box attached) and keep your mpg high and job done. I think performance cars are gotten just a little bit too easily nowadays and many younger drivers crash them due to lack of driving experience, but hey maybe thats the old cynical guy in me coming out.
And as Dan said above, insurance is not something you try to trick or get around, its about putting forwards an honest assessment of your car ownership. If this second driver never gets in the car, you are committing fraud.