Whether to use packaged soundproof drywall, also known as pre-damped drywall, is a great question. With “soundproof” drywall available at big box stores and drywall distributors, anyone would be nuts to not at least consider these products.
First, let’s consider that any pre-damped drywall is simply two sheets of standard drywall or cement board laminated together with a damping compound. No mystery to what they’re made from.
Now let's look at this more closely. With sound isolation, there are only four elements of soundproofing, and you can learn more about that in this article: The 4 Elements of Soundproofing. In brief, the four elements are decoupling, absorption, mass, and damping. With drywall, we are only discussing potentially two of the four elements, specifically mass, and damping.
Five factors to consider when choosing between pre-damped soundproof drywall or building your own
#1 Mass- When looking at Transmission Loss (the reduction of sound from one side of a wall to the other), mass is a very large factor. The heavier wall will be harder to move and will resist sound better. A sheet of standard 5/8" drywall from Lowe’s, etc weighs 70+ pounds per 4x8 sheet. Two sheets give you 4.5 pounds per square foot. Nice mass. A typical sheet of pre-damped drywall may weigh 2 pounds per square foot. The large difference in mass directly translates into a large difference in low-frequency performance.
#2 Damping- High performance damping compound isn’t cheap. Generally, the less expensive the pre-damped “soundproof” drywall is, the less damping material is present, and obviously the lower the performance. Yes, even using a very small amount of a damping compound can technically reduce some quantity of the resonance inherent to the drywall. This is a good thing. However, from a marketing standpoint, a drywall panel can be called "damped" even with an extremely small amount of damping material. There is no standard or guideline for how damped a panel has to be before it can be called "damped."
#3 Performance- See a summary here: Green Glue Company Data $7 a sheet standard drywall + Green Glue performs better than even high performance pre-damped drywall, with the exception of a “THX board,” which is 1 1/4" thick, weighs 200 lbs, and retails for almost $200... a sheet.
#4 Waste- There is an average 12% waste factor with drywall work. You can fill the dumpster with $7 a sheet drywall or $80 a sheet pre-damped board.
#5 Cost- To get remotely similar performance (see the previous link) you'd have to move up several levels beyond the opening pricepoint Lowes “soundproof drywall” board.
The bottom line is that you will always be able to field install a much heavier, much cheaper and much higher performance board than any pre-damped board available. This is true even when we factor in the labor to install the wall.