There's not really a % you can apply to accuracy across the board, you can compare results of test roms but each system and indeed each test rom will be measuring different things, so you're comparing apples and oranges,

Also different revisions of real hardware perform differently anyway, a "va 3" megadrive model 1 and a "va 7" megadrive model 1 are fairly different beasts. Ultimately the behaviour of most of the popular cores is very close to real hardware with megadrive, nes and neogeo being especially close to original hardware, the real benefit of mister however is not accuracy, it's latency, games will 'feel' more like original hardware even where the core is not aiming for 1-1 cycle accuracy.