Splitting/Attatching - a few years ago one pair of Sat morning trains *didn't* combine and ran separately to VIC the first just ahead of the normal path, the standard hour timetable is designed to allow this both ways though one train may not be able to call at Clapham Jct. SN now use one path for the third Brighton fast not calling at Haywards Heath where it passes the Coastway service while it's dividing/attaching. SN also wanted the other path but FCC's objection was upheld by the Regulator.
Early morning trains do run at different times in the mornings and arent the same as the clockface timetable through the day. 8 or 12 car units which may be stabled at Littlehampton or Eastbourne may of needed to be ran from only 1 of the stations to Victoria, in order to run a service back down, which would split. Its likely its changed though if these services no longer run, stock may come ECS from Selhurst Depot to form Coastbound services.
Another advantage of portion working is reduced crewing costs and possibly track access charges. Disadvantages are up-to 10 min time penalty, passenger confusion with some going the wrong way and on busy 12 car trains overcrowding in the 4 car portion. From VIC part of the problem is the way pax going no further than Hay Heath don't care about portion lengths causing unnecessary crowding of the 4 car for those who need to be in it.
Can cause confusion of course, thats why there are frequent announcements, problems occur however when passengers ignore the announcements.
16-car trains - were being considered in the '70s for non-stop Brighton fasts, I'm only suggesting it at principal stations on the BML. VIC P12> could handle them but may need a bit of tweaking now, Brighton could be done by restricting East branch access. Clapham Jct, Gatwick & Hay Heath all have room for extended platforms. E Croydon will be more difficult though operators have been asking for more flexibility ever since the present layout was done so a possible BOGOF?
Victoria is one of the stations that couldnt be extended as the extensions would stretch into the throat of the "fast" platforms, would block the slows as well if you extend 12/13. P19 could possibly be done though with a bit of work maybe. CLJ could possibly done with a bit of work, Gatwick couldnt because of the trackwork, HHE could be done and BTN could never be done because of trackwork and because Lovers Walk Depot is in the way. You cant extend platforms which then block of the Coastway East line as it would effectively force the closure of the line !!
Gatwick is getting a new Dn Fast Loop platform, don't know if this will be a problem but it needn't be, might have to take a bit of car park. Other problem is power supply...
It might mean an extra path on the Down Fast which could run fast through Gatwick overtaking a stopper, possibly could be the same for the Up Fast too, no doubt would the path be used by a BTN service though. Power supply would be a big problem though as you say, i think a 16-car 377 would be thirsty to say the least !!