Maybe they'd been better off order an AC version of a 444 at the time. What's the stopping pattern like on the line? The stopping pattern might have pushed them towards going for a "commuter" door pattern (1/3 and 2/3 doors)
Yes of course I like the 309s and their ilk for journeys over 1 hour, (the clue is in my user name), nostalgia is a wonderful thing but times have changed. When I used them (early '70s), the bulk of the mainline commuter traffic was handled by 307/308s which provided the core Chelmsford, Witham/Colchester service. The (relative) luxury of the Clactons meant that some travellers crammed on them at Liverpool St but in reality the corridors/gangways quickly clogged up, and their dwell times were tedious. In the peaks, they either took paths that effectively restricted them to the maximum speeds of the outer suburban trains, i.e. 75mph or the way ahead had to be cleared allowing them to reach 90-100mph beyond Shenfield, which was wasteful for a realtively few passengers.
As usual, it's all about the average passenger journey. Even today, the volume of Clacton passengers is low by GE standards so providing low density stock would be unlikely.