I agree that it would be "not fatal". Would it be good for Britain... no. I can see where he's coming from with the "shot in the arm", but that seems like it would be a rather short-term advantage, and more a matter of perception than anything else. The disadvantages in terms of lost influence and lost business would be real, though. Yes, Britain has the Commonwealth to fall back on, but still, the EU is a rather bigger force than that.

I kind of suspect the Conservative leadership is just playing an elaborate charade here, to be honest... kowtowing to the Eurosceptics helps them keep their core voters happy, in a way which doesn't antagonize other potential voters too much, while strengthening their negotiating position for future European agreements, since the EU doesn't want to lose Britain. But they have enough ties to the business world to realize that actually leaving could seriously hurt the economy.

Cameron can't come out and say these things because he has to deal with his European colleagues on a regular basis, and because he has to look responsible. But someone like Johnson, among the most prominent people in the party and with something of a reputation for being a loose cannon anyway, that's perfect.