After an injury I went down to 12 lbs for 1 1/2 years.
I still do practice with my 12 lb stuff.
Took it to a sport shot dual pattern tournament last month and cashed, got $350 back. And no one would have thought I was throwing 12 lbs.
The idea that a heavier ball hits harder has been debunked by Art of Bowling on youtube, and in the book, Bowling Beyond Basics.
The power the ball has is weight X (speed)squared
So a small increase in speed, makes up for loss in weight.
Of course getting the ball into a good roll and not throwing it past the break point matters. If someone throws a lighter ball too fast - that won't work. If someone says dropping a lb or two makes a big difference, then I just wonder how they are throwing that particular ball? Are they throwing it too hard, past the break point so it doesn't get into a good roll. Or if someone really believes that ball is too light, now they have an excuse for that 10 pin. Nothing new about bowlers having an excuse for why some shot didn't strike.
Yes, you will leave 10 pins with a lighter ball. You'll also leave them with a 16 lb ball as well.
Even if the 13 lb ball has less power, I'm sure having a ball that is easier to throw, leading to better accuracy and consistent release, will more than make up for weight change.