What Kelsier describes as "pushing with your own weight" could also be seen as a primitive understanding of inertia: heavier objects are harder to accelerate (or decelerate) than lighter ones. If you step on a skateboard (to reduce friction) and throw a basketball away from you, you will be pushed back by the force of your throw (Newton's 3rd law states that while you push on the ball, it pushes on you with equal force), but you will go much slower since you are much heavier.
This is, at least to me, exactly what happens in Scadrial : as a coinshot's body is much heavier than the coin, the reciprocal force (assuming allomancy doesn't violate Newton's 3rd law) will only have a small effect on the user's body. As such, the coin gets shot real fast while the coinshot only suffers a little recoil.
Also, in the books, Vin often attempts to push objects using all her allomantic might, even using duralumin to enhance her push (thus exerting more force). At these times, she is often described as bracing herself on heavy metallic objects (or metallic objects fixed in a solid structure) to maintain balance (i.e. not being thrown off by the reciprocal force). As a full Mistborn, Vin also uses pewter to enhance her body's capacity to endure crushing pressure to exert even more force while pushing objects.
In most cases, I believe that a physically fit coinshot could throw coins out at deadly speed (from his center of mass, not his hands) while only suffering light recoil, which his body should be able to absorb. And as long as his body could resist the strain and the pressure, he could brace himself on structures behind him to push coins even faster.