Powershifting is definitely faster. Any time spent shifting is time not spent accelerating.
There are two different types of shifting used in order to enhance shifting performance. They are "powershifting" and "speed shifting", and both have their advantages/disadvantages.
Powershifting is where you do not lift off of the gas during the shift. This is the absolute fastest way to shift the trans, but it's downfall is that it is very hard on the powertrain. This begins with the clutch linkage/cable and continues all the way back to the tires. Synchros and gears in the trans can be damaged, u-joints may fail, axles could break and the tires get worn. This can be amplified by using slicks on a higher powered car. Another less common but just as important flaw is that a missed shift can result in damage being done to the engine from overrevving it. If used in moderation, this technique is very good and will definitely cut your e/t. I powershift 95% of the time at the track and have noticed at least 2 tenths drop in e/t.
The other method is speed shifting, which is much less detrimental to the drivetrain, yet still very quick. To speed shift, you let off of the accelerator momentarily while shifting, and immediately reapply full throttle as you are releasing the clutch. This allows the "slack" in the drivetrain to take some of the jolt from the shift, and full power is not being applied when the clutch is released. I've noticed about a tenth reduction in e/t. When I am messing around on the street, I always speed shift instead of powershift...Unless I hear a ricer blowoff valve on whatever I'm racing...
Sorry for the long post, but I like to be specific.
J.P.