This is a good question/topic.
We're always told that we can always improve if we work hard enough at something. But with smash power, there is a limit to what each person can physically achieve no matter how hard you train. Although power helps a lot, it is not the only thing that can make a smash effective.
So my first piece of advice to those who find that their smash power is lacking or at their limit is to work on accuracy and placement. Really make sure you can place your smash anywhere at will.
Whilst we all recognize FHF as one of, if not the best smashers of our sport, his smash was not actually that efficient (in terms of benefit to effort). It was reliable in that he was not only able to hit hard anywhere on the court, he was able to sustain it as well, he was relentless and this often forces opponents into error eventually. But his placement and variations were very limited and this made it quite predictable although it was still difficult to return due to the speed.
Other elite players, particularly those who have come to the fore in the last few years make far better use of angles, placement and variation instead of just brute force. This is a far more efficient way of forcing errors.
When we look at how opponents make errors in detail, we are essentially trying to make the defender mishit or mistime their defensive stroke (or in some cases both leaving the shot to the other). To do this we can try the following with our smash:
1. Speed - hard smash, reduces defender's time to play quality defensive shot.
2. Angle - using steep smashes does several things. It prevents counterattacks, moves defenders forward and makes the straight defensive shot more likely. Angle is important as there are two areas you are trying to hit - either as high as possible on the defender's body (but making sure it is not long of the baseline) or very steep such that the defender has to reach forward to get it. But ideally not between the two where it is reasonably easy to defend and counterattack.
3. Placement - place the smash at the body in an awkward position to defend, place in-between the defenders for confusion, or place wide near the sidelines. This is perhaps the most common smash that if you know how your opponents defend, can be very effective. The ideal position on the body is just above the racquet arm elbow area. Watch how your opponents defend, look at how they stand, where they stand, what grip they use, how they play the defensive strokes and where they like to place the shuttle. Knowing these things can help you decide where to place the shuttle.
4. Variation - This is to prevent your opponents getting too comfortable with defending your smash and keeps them guessing and moving.
The most efficient smashes are a combination of these attributes. The more we move the defenders, the more they are likely to make an error, even if it is small movements such as having to change grips all the time or to change from forehand to backhand or having to move forward and back to base. If you can catch your opponents by smashing at them while they are still trying to move back to their base position this often produces some error which you can build on.
As mentioned above, if your smash power is limited, try working on the other points above, particularly 3 and 4.