Pretty sure game cartridges tend to have an upcompressed bootstrap loader and then much of the cartridge data is heavily compressed to save on cartridge costs. Optical discs have fixed storage capacities which enables more wasted space especially if its a game that easily fits on the disc. It can have optimised loading to help speed up loading. It's easy for optical discs to be faster than cartridges if heavy compression is used on the cartridge.
I don't think its realistic to think of cartridges as an extension to main memory because while the boot strap loader may not be in main memory there is probably uncompressing routines added because of being a cartridge. I'm not expecting to see much difference between cartridges and downloaded games plus also if the same game is sold digitally it will have to fit within 4GB of main memory anyway so there can be no advantage to the cartridge game.
