You should be able to smash or loop at near full power without going off balance. Top players can play great shots in rapid succession because they are always balanced, which leads to a rapid recover for the next shot. When you go off balance, even slightly, you cannot recover quickly for the next shot. There are two times when players tend to go off balance: either when moving to the ball or when following through.
When moving to the ball, if you keep your weight somewhere between your feet, you can stay balanced. If you instead lean or reach toward the ball, you go off balance as your weight goes over just one foot or even beyond. To see this, stand in a ready position. Lean to your right (or left). Now try to move to your right (or left) - see how difficult it is? Your first move needs to be a step, not a lean.
The second place where players often go off balance is when they follow through too much to the side, especially with the forehand. If a righty has a big follow through to his left, he'll be off balance and unable to recover for the next shot. Instead, imagine a pole going through your head, and try to rotate in a circle around the pole. You don't have to do so exactly, but if you focus on rotating around this circle more and less off to the side, with your weight between your feet (or at most over the front foot), you'll be ready for the next shot. Even with a complete weight transfer from the back leg to the front leg you don't need your weight to go outside the feet, which puts you off balance. Note that a wider stance makes it easer to generate power without going off balance.
Here's a test on whether you go off balance in your follow through. Shadow practice doing your most powerful forehand shot, whether it's a loop or a smash. Freeze at the end of your follow through. Is your weight over your left foot (for a righty), or has it gone even slightly beyond that, leaving you off balance, requiring precious time to recover? Have someone give you a light shove and see if you are truly balanced. You should finish the shot balanced and instantly ready to return to ready position to follow with another shot. The best way to practice this is with multiball, where a coach feeds you shots in rapid succession, and you are forced to recover quickly for the next shot over and over.