This is an interesting post.
Polling Raspberry Pi GPIO is very slow,< 1mhz and funnily enough RPi B+ can poll the GPIO pins much faster than RPi 2 or 3. However your idea is very interesting, I have had moments of madness where I thought it would be fun to interface directly with the cpu for various reasons. But it always seemed liked more work than I was prepared to do. I however have a suggestion for you that might work...
1. You don't have to "find" the high scores etc. in the cpu, because the machine tells you all you need to know on the display!
2. Intercept the display data.
3. identify the data you want, like the high scores.
4 send the data to your pc/rpi or whatever.
The previous post, the ram idea is also interesting, but more wires, might be cool to play with if you had an extra wpc cpu to play with, maybe some soldering ... Also probably not needed to clone the ram, but see how long you can delay comms between the ram and the cpu before it complains, to give you time to read your addresses ..