Regarding the comparison between a 747 on 3 engines and an ETOPS aircraft being made by Rugerdog and Cerberus.
An additional factor to consider is that, all else being equal, a 747 on 3 engines is 50% more likely to have an engine failure than an twin (since it has 50% more engines operating - 3 vs 2).
Consequently while the performance of a 747 on 2 may not be a lot worse than an twin on 1 - the 747 is 50% more likely to end up on 2 (when it is starting with 3) than the twin aircraft is to end up on 1.