Honeslty, I've found a setting that is quite good for both 4 and 8 way. It is not perfect however. every now and then, I hit a wrong direction on Ms.Pac Man.
So I too am interested in getting this done. It should be pretty easy. First find the perfect setting for 4 way, measure the pot's' resistance (needs to be soldered out first 1).
Then back in again and measure again for the 8 way setting.
Than make a switch and use resistors that are as close as possible to the measured values, or use a second pot.
In an ideal world, the software (mame/mamewah) would know if the game is either 2,3,4 way or 8 etc. and switch over the stick resistor value automaticaly (using a small relay).
I can do the electronics stuff, but not the software part.