Your estimation is way off. The -400ER is a 5,600nm aircraft. The B787-8 will be a 8,5000nm aircraft. (You might want to compare the OEW difference betwwen the B777-200A and the B777-200ER.) If the OEW of the -8 is 20% lighter than the -400ER, then Boeing would probably need materials that would be 40-50% lighter than today's aluminum!
Its no wonder the why A330-300 wiped out B777-200 for regional flying.
Wipe out is an exaggeration. Before the B777 was launched, Airbus already had 120-130 orders for the A330-300s. Neither the A330-300 nor the B777-200A has been a hot seller for the past 15 years. The A330-300 picked up some pace after Airbus started offering the 230t version of the aircraft.
The 787-800 is about the size of a A330-200... Fuel capacity will most likely about 37,000 to 40,000Gal range.. that would most likely mean a large center fuel tank which might eat into cargo capacity.
THe B787-8 is smaller than the A330-200, but it will have about the same cargo capacity of the A330-200.