There is noticeable improvement.
There are exercises for helping with lower abs. Have you tried the simple 'crunches'? As well, go to a chin-up bar and do leg raises. These help a lot.
I always incorporate a set of sit-ups where i keep my back and neck completely aligned (Straightened) with my hands behind my head.
I use as much lower abs to bring my upper body to an upright position.
You'll notice a difference.