My ESL has excellent dispersion and achieves this without curving the panel. One cannot stretch a diaphram in a curved panel without making many comprimomises. If this needs further explanation let me know. There is indeed a way to do this but it requires wide electrode spacing and high polarizing voltages.
The popular ESL, whose name I need not mention,has a high crossover point (700 Hz typically) and thus is really a two way speaker with a cone woofer and ES tweeter.
Roger