afaik escorts gets destroyed by one critical hit. that means, that all the 400 hull escorts die more or less after the same amount of firepower compared to 200 hull variants. that is because the probability for one crit during the first 200 hull damage is very high. a better design should have different average time to death tied to escort hull points. one possible solution for this could be the reintroduction of "hull breach" as the crit suffered by escorts. it destroys a fixed amount of hull points so that 400hp escorts could survive more crits before they are destroyed compared to 200 hull escorts.
another issue imo are ships with the "immunity to crits" trait. they are too tanky with no loss of efficiency until they are destroyed, which means they tend to overperform. kroot balls for example are extremly tough in their braced stance, shrugging of enormous amounts of fire. I would prefer if their trait gives them a percentage reduction of receiving crits in general, subsystem and weapon immunity to crits and possible hull breach and fire crit effects instead. that would mean still no loss of efficiency until destroyed, but faster loss of hull through hull breach crits against very high amounts of firepower (compared to damage needed to get crits against ships without the "immunity to crit" trait).