Don't judge anything off Skipper's models as they were generally built by eye, rather than from drawings. His 5700 class is definitely overscale and out of proportion in a number of areas.
The new 5700 is the same length as Gary's 8750 class (which will be accurate) and close enough as makes no odds at buffer height and tank top height.
The chimney's a bit low, and the dome a touch big, but otherwise it's pretty accurate I think. The Churchward cabs were lower than the Collett cabs, so that's correct.
Panniers are a complex subject - there were lots, mostly rebuilds of older locos - but the 5400s were bigger wheeled and unrelated (by Swindon's standards) to the 5700s, despite the overall dimensions (length, boiler pitch and overall height were within an inch or so. They were smaller though in respect of a much lower tractive effort than the 5700 with a different, lower pressure, boiler and smaller cylinders.
5700s were intended primarily for shunting, 5400s were for light branch trains where the extra turn of speed and freer running allowed by the bigger wheels might come in handy.