Correct, Ejecta, same as CocoonJS, loads all images (but not audio files) on the initial loading of the game.
I just tested my game with the new ejecta exporter and the results as basically the same as with cocoonjs when it comes to memory usage. Note that if you used Ejecta in the past this exporter won't bring anything new to the table.
As for the performance, on the iPhone 5 the game runs smooth both with CocoonJS and Ejecta, maybe a bit smoother with Ejecta but maybe it's just me. On the iPhone 4 both versions have a harder time keeping the framerate up but the CocoonJS version seems to do a better job at it.
Regarding memory management, after the game loaded successfully the memory consumption actually drops, so in my experience there's a good chance that if the game fully loads without crashing, even with low memory warnings, the game probably won't crash while running.