4w/kg at 75 is 300w
4w/kg at 70 is 280w
That's 20w difference on a very typical group ride difference in weights. Not even likely the largest difference.
I ride with folks sometimes where the difference is going to be more like 10kg's from myself to the largest rider. With that in mind the math becomes:
4w/kg at 70 is 280w
4w/kg at 80 is 320w
A 40w difference.
I'm guessing here several riders are in the 5 to 10kg lighter range.