Hi marosmith -
I will admit to a huge personal bias here. Not a big fan of stiff racquets. My brief stint with the 2012 PD last summer left me with my first ever case of TE after 30+ years on the court. But I will do my best to answer the question squarely. If you want to generate more ball velocity, you have two choices. You can swing faster, and a lighter racquet will help you do this, or you can swing a heavier racquet the same speed. (velocity = mass times acceleration)
Light Racquet, Fast Swing:
Swinging the racquet faster has an added bennie in that you will gain additional spin out of the bargain. However, if you are swinging so fast that you pop out of your comfort zone and start making errors because you are swinging wildly that is not good. The other problem is that when making a racquet very light, it becomes unstable. One way to counter this instability in the lower swingweights is to make it a bit stiffer. Take a popular low sw frame like the 2012 PD for example. I can't think of any frame that is more stable at such a low SW of 308. If you compare the PD to something like the Donnay Gold 99, which has a super thin 16/18/18 mm beam, it will give you a bit more juice at very the top of the hoop for 10 less sw units. It's kind of a wash everywhere else. You could look at this like "free power" since the PD is easier to swing at 308 vs 318. However, if you compare a 2012 PD to a really thoughtfully engineered frame like the Becker Delta Core London, or the 105 Red, You'll find that those racquets have similar power everywhere except for the very tip, for the price of just 3 more SW digits. So the question becomes, is that little bit of "free power", a good trade for reduced arm comfort? For me, not in a million years. For somebody else, maybe yes. Is the PD worthy of the "Rocket Launcher" moniker? Nope, not in the slightest. It's complete myth. Not a "Rocket Launcher" by any measurable, comparable means. Some free power, but not much.
Heavier Racquet, More Flex, Same Swing:
you wont need to swing quite as fast, which means that you might be able to control the racquet a bit more and reduce unforced errors. Your big bennie out of the bargain will be added stability on off center hits, which you will experience as a bigger sweet spot, and additional arm comfort. A reduced swing speed will not do any favors to spin production, but high sw also implies more dwell time which is spin friendly.
My last image showed four very different flexes and swingweights. The one below hints at the incremental advantage gained with a stiff racquet, when the SW are very similar.