Clouds of black smoke once you've f***ed the very expensive emissions system and thats assuming you don't get stopped by the police.
There would be no more bore wear than a smaller engine. Do you think truck engines have to have their bores relined every few years?
As for fuel consumption, selective disabling of cylinders can help with that and if the block is aluminium a V8 will weigh about the same as an iron block
4 pot.