"Another candidate would be the GS3 and
GS6. They are essentially the same, with variations in the skirting and appliances. "
Actually not even close, the GS-3 is essentially the same as the GS-4, boiler, drivers (80"), cylinders, etc. The cosmetic differences are the cab and boiler front.
The SP GS-6 and the WP GS64s are copies of the earlier, smaller GS-1 and GS-2 classes, engines with 73 inch drivers, with an all weather cab (like the GS-4).
It is interesting to speculate on what the next "quality Plastic" steamer should be. Almost as common as the consolidations, for an equally long period were the Ten Wheelers. A nice size for any layout, I like SP but what would be the best prototype 4-6-0 for a P2K model?