The GAP issue concerns me more, car gets nicked / written off and I could be £10K+ in the red, much more likely and more costly than an new engine IMO.
From reading on here 99.9% of warranty issues are electrical / squeaks / rattles, no engine failures as far as i know. Only the known turbo failures on 335i's which are a straight replacement - no questions asked.
But you're right - the risk is always there
E63 AMG here at last!