Friction on the leaves will slow the compression and rebound of the spring. But overall yes, a really soft leaf will do the same as soft coil. Its just that leaves that soft have all kinds of problems. Because a leaf springs also controls the lateral movment and rotational movement of the axle, control arms would then be needed. Then you'd have to buy some nice shocks to tune up the suspension and by that point you're pretty much at a 3link with coilvovers.