I get only about a 50 rpm drop on applying carb heat during the run up - used to get about 200 in 172's.
I too have one those carb heat gauges with the small yellow avoid area in which the needle always seems to obstinately remain embedded.
Things would have to get quite desperate for the extra heat of leaning to have an effect - but if all else fails, why not - as the TV ad says "every little helps".