Raising or lowering the motor to correct steering issues? I have never heard that.
What generally causes serious steeering issues is torque steer (What John Stiles is referring too) and off-center engine mounting.
As you have tried to correct via moving the trim tab, you know about torque steering. You can only go so far with that tab before you cause more problems than help (and cut efficiency WAY down).
One thing to check - go out on the most calm water you can find. How hard is it to get the boat to track straight at trolling speed? Take it up a notch (but not enought to get up on plane) - still pulling?
And finally, get it up on plane, but not wide-open. Does the steering problem drastically increase as speed increases?
Could be a lot of things - I really would double check to be sure the motor is mounted square and dead center on the transom.
A change in prop may decrease the felt torque, but will be a sacrifice in other areas (top-end speed, RPMs on motor, etc.).
When you trim the motor (it does have power tilt/trim, doesn't it?), does that change how much it pulls to the left?