So would you have used an 8" OD spider with a 4" coil? Thats about all you can do with a 12 spoke basket. Ask RE Destroyer owners what happens when you do that. Im pretty sure this is why Rusty didnt use a 4" coil on his stuff.
Just because a sub has a larger diameter voice coil doesnt make it better, not even close.
Or... Rusty could tool a new basket... whats $70,000 when you can have a 4" coil?
Edit: Not trying to argue with ya man, not at all. There's just a whole lot more to it than adding surface area for better heat dissipation.