If the 165s retain 5 seats across and the 166s are 4, then surely, once the public realise the difference they will head for the 166s - especially if they have working air cooling? The problem could then be that the 166 section becomes full, the train moves off and the overflow of passengers are not able to access the spare capacity in the 165 section of the train due to lack of through gangway. Perhaps at the next station, overflow passengers will try and hop off and back into the 165 section?
165s are in the process of being fitted with 'Air Cooling' (165110 &165111 being the first, currently at Wolverton).
166s have 'Air Conditioning' which has always been troublesome.
Most people will ram into whichever part of the train has stopped closest to the platform access point :roll: