Unless i'm mistaken I don't think this speed trial is to increase sector capacity.
I was under the impression it was to cut down on 'bunching' of LTMA outbounds. Also from a safety point of view aircraft flying at slower speeds are more manouevrable etc.
As a LTMA controller I have found that because aircraft are flying slower they remain in the sector for longer so you actually end up talking to more aircraft at any one time.
Despite what people may think, before this trial we didn't always lift the speed restriction on every aircraft. Being able to keep an aircraft at 250kts or allowing it to speed up is a useful tool that we no longer have at our disposal.
I also do one of the TC 'feed' sectors and have found at higher level this trial has made absolutely no difference and we still need to put aircraft on parallel headings to maintain seperation.
Aircraft such as 747's need 270kts minimum which we permit. This though means if left they will catch the preceeding aircraft that is flying at 250kts (We obviously don't allow this to happen, by accelerating the ist aircraft or by using headings etc.)
Just a few personal observations!