Not sure how the number of seats compare, but the current 9 car trains are often overcrowded in the peak. So I suspect the the 387s will be worse, at least until Cross-Rail opens. Once Cross-rail opens they will lose a lot of passengers so the 8 car 387 will be fine.
** the loss of passengers because cross rail goes to useful places, such as direct to the city. I can't image anyone head to Bishopsgate will want to pay a forturne for HEx and Taxi when they can get a direct train, likewise anyone going further afield can get to more changes with a single change**