Masi, You are correct regarding momentum, however while the two sides are no quite as lopsided, I still stand by straight throwing with slightly lighter rocks at a much greater velocity being stronger. The sling is user dependent, and if the slinger is putting more energy into the throw, it should be a stronger throw.
Increasing the angle of a straight (hard) throw for distance doesn't make it lobbing, it is just straight throwing on a different angle (like if you were trying to sling straight at a bird or an enemy soldier on top of a castle). Lobbing is using unnecessarily heavy ammo, aiming high, and throwing with less energy (I'd even say weaker if I wanted to be derogatory, but I won't

) in order to drop the shot in on the target in a controlled manner.
The historical slingers that utilized 2oz lead glands slinging near 400 yards definitely weren't lobbing, but they did need to increase their straight throwing angle in order to achieve maximum distance.
I would encourage others to sling the same way as it is the most versatile way to sling while maintaining accuracy and great power.