[jdk81]

You could make the cable wheel elliptic so it only moves rapidly near the end.

[mazdaspater]

Put a Goddamn wideband in. Techedge in Canberra do a good cheap one.
Stop guessing. Start improving.

You can make a circular throttle wheel, with a non-centred hole in it. So the last half of throttle travel goes very quickly. You just have to figure the radius you want at the start and the radius you want at the end of travel.
You can make one out of big washers and a welder (or Pop Rivets) if you want proof of concept.

Sort Throttle out first, tune it second.

And put a wideband in.