I'm still using ES on turn 80 in my Pirates game - basically they're support ships now, and just move in to capture ships taht my LCs have disabled.
Their small size makes them no threat to the AI, and so they don't get shot up as much. Also, since they have no shields, there is no penalty for making a boarding attempt
When funds are tight, the ES is viable, but I'm probably not going to build any more, and switch over to BC attack ships & LC support.
Perhaps, instead of increasing the size of the ship, you should give it an inherent movement bonus for its small size?