I think you're probably correct or at least more correct with your 60% figure. The diameter of the RS5 measured across edge to edge is 4" flat. That's fine, but the driver itself isn't a flat circle. I'm then assuming the stated 4.5 " driver size is determined based on the slope distance from edge of cone to the center of driver. ( measured on the real face of the driver itself) So that distance could very believeably be 2.25". Now if this paper thin theory is somehow correct, the slope distance of the 8" driver edge of cone to driver center would simply be 4". The 2 radii are now 2.25" and 4". That would equate to 3.16-RS5 drivers = 1-RS8 driver. I concede, but more importantly, time for a beer.