E retention is a combo of a few things: Momentum, airframe aerodynamics, and yes... engine power.
Take whatever airplane you want to test, take it to 10k, dive to 450mph and level off with with normal mil power. According to this tinky dink spread sheet I have that Luche provided the Mossie has the best E retention. While the Mossie does in fact have an awesome ability to retain E, this data was made with engines off.
Trying to guage the "E" retention of an aircraft without taking into consideration the engines is like pushing dragsters down a hill with engines off and whichever one gets to the bottom first has more "power". The engines are a very big part of how well an aircraft can maintain the extra E, dont kid yourself.