Buscando buscando he encontrado una teoría distinta de porqué es mejor pantalla gris que filtro ND:

anyone using ND filters to increase black levels on entry projectors (hd20) - Page 2 - AVS Forum
I've experimented a bit with filters some years ago. Basically, I think that if you need an ND filter, something is wrong with your setup. All the filter does, is decrease light output. You are MUCH better off doing that by choosing a darker screen, if you actually have too much light output. The reason for this is that in most cases your room WILL have an impact on black level. An ND filter will not decrease the room's impact on black level, but a darker screen will. Hence, no filter + dark screen has higher contrast than filter + bright screen.
Resumiendo: la pantalla gris ademas de bajar los negros aminora los efectos negativos de la luz rebotada de la sala.