I think it's going to vary depending on lots of things, like OS, Mame version, possibly different between USB and serial Optipac as well.
My hardware: 700mhz Athlon, serial Optipac with USB upgrade pack, Oscar Pro, under Win2000 Pro
My settings: Tempest - Dial Key/Joy speed: 20 - Sensitivity: 25% -This seems to be a good mix of speed/sensitivity but it's just my taste. Haven't seen Tempest in an arcade since 1982.
Arkanoid - haven't tried it since I got the Pro, sorry...