I'm curious to know why the new points from the DSS to the DSF are 50mph and not 60mph. Both pieces of track are parallel and there's plenty of room to have a longer, quicker crossover.
10MPH difference? Maybe they would have put in a 40MPH turnout if they could have got away with it but in signalling that would require approach release or some other complex advance aspect sequence with splitting distants, preliminary route indicators and the like. Where these are close to other junctions it starts to get really complicated. You can't for example have flashing yellow sequences for one signal overlapping with the same system approaching the next one, so you have to choose splitting distants or PRIs. Make all the diverging differentials no more than 10MPH slower than the fastest route and forget all that. You can have free aspect for all routes.