I wrote the article that Roger refers to.
http://www.geocities.com/roger_forgues/Boost-tabs.html
"Labor Saving Devices for Overworked Servos, by Carl Risteen. It is still in print, in Air Age's "How-To's, Vol. 2. One nice thing about boost tabs is that they can serve as ground-adjustable trim tabs, to help trim out those inevitable small misalignments and warps. These show up in the form of a model that is in trim over only a narrow speed range. Trim it at half power, and it goes out of trim at full power, and vice versa. I am sold on trim tabs, and frequently use them with a linkage that makes them perform as boost tabs as well. I find that boost tabs can lighten servo load by up to about 85%, before non-linearity gets objectionable. I set their travel quite low at first, and gradually increase travel until I get just enough boost to do the job - too much can be very bad, forcing the servo to work in reverse, trying to restrain the control surface from deflecting on its own. The usual evidence of this is that the model refuses to trim out, due to linkage slop allowing the control surface to move a bit off center in each direction.