I can't confirm from first hand experience, but I've heard a number of anecdotes which suggest one or two ex-Littlehampton trains - already 12-car - are full and standing from Hove outside the school holidays. I suppose, with people interchanging, some/all of the standees might grab seats at Haywards Heath, Three Bridges or Gatwick.
It does seem likely the arrival of the 700s will result in more standing from Haywards Heath (45 minutes), if not further out. I'm East Coastway, and I know there would be standing from Lewes (60 minutes) on a few trains were they replacing 377s/442s 'like for like' here. But we had a lucky escape.
I wonder whether this decade is 'peak London'? They do have Starbucks and Waitrose in Birmingham, and a stroll into the city centre from a nice house in Edgbaston sounds more appealing than a silly-o-clock alarm call and strapper-hanging from Burgess Hill to City Thameslink.