The problem with ED affordability and chasing merit aid is that many families whose EFCs would make them full-pay, nevertheless might want/need merit to more easily afford for their kids to go. Getting out of ED for lack of affordability is based on EFC, so breaking the agreement for lack merit goes against the spirit of the agreement. (Don’t get me started on the legitimacy of the whole idea of ED, as I think that it is inherently unfair to students, but that’s another matter altogether).
FWIW, most schools will tell you that if you want to compare offers or if merit aid is an issue, do NOT apply ED. EA is fine, though. Whether applying early matters or not vis à vis availability of merit probably depends – many schools have early submission dates for merit consideration; others require EA for consideration; others claim that EA/RD are considered equally.