That can happen, but doesn't HAVE to. If you install weights which are so light, that they cannot overcome the spring within the effective RPM of the engine, then you are right. In that case you would need a "softer" spring. But let's assume that we are keeping everything else original and just reduce the weights about 10%. If at the original set-up, the sheaves are fully squeezed together at 65 kmh, now with the leighter weights this state is being reached at 70-75 kmh (?). From that point on we are traveling again at the top transmission ratio, independent of the weights, all the way up to the same(!) top speed. Actually, you can get the very same effect of increasing acceleration RPM if you keep the same weights and just stiffen the spring.