I noticed last night with my momo that pole position plays like crap with a 270 and the shifter is high low in only one direction as ooposed to up down like the real game.
Pole position used a 360 degree wheel in the arcades, and 270's suck at controling these games. Most "behind the wheel" driving games use 270 wheels, but PP & PP2 are two of the few that used 360 wheels. These games will play better with spinners than 270s.
Here's an old-ish (0.62) table of driving games with 270 vs 360 inputs:
http://urebelscum.speedhost.com/drivingInputs.html The 270 games should play fine with you momo wheel.
what you have noticed works and doesn't work as far as shifting.
Most games with hi-low gears are hacked in mame for easier keyboard play. These game use toggle like buttons to switch from one to the other, while most original games high gear was equiv to holding down a button. A quick hack would be to map both mame's high and low gear to both high and low gear input; if the device and the game get out of sync, pause the game, shift back into sync, and unpause the game. The cleanest solution would be to edit the source.
Gears with more than two gears are simulated different ways, with different hacks/fixes. If mame driver uses a "shift up"/"shift down" hack, you could map first gear as shift down, second as shift up, and driver with the gear in "neutral" when not shifting up or down.