As Cyclone03 had mentioned before, I think you're talking about two separate issues. DAMPENING, and BALANCING. Yes, they're both important, but they're not the same. In an internally balanced engine, the dampner is still needed, but it's only used for DAMPENING vibrations, which otherwise would travel through the crankshaft, and into other components such as the oil pump gears. The vibration Dampener isn't used, nor needed for balancing on an internally balanced engine, which is already balanced BEFORE the vibration dampener is installed.
On an externally balanced engine, the engine internals are NOT completely balanced as an assembly until the flywheel, and vibration dampener are installed, because the dampener and the flywheel are both used to balance the crankshaft. As for heavier vibration dampeners being used on internally balanced engines, you're correct. The car manufactures are going with bigger and heavier vibration dampeners in Hi-perf applications, yes. But that has nothing to do with balancing needs. That has to do with the fact that lighter vibration dampners often do not absorb, or "dampen" the vibrations that run through the crankshaft as thoroughly (especially in a high RPM enviroment).
Those vibrations are a normal occurance in any internal combustion engine, and will always be present regardless of whether or not the engine is balanced. The vibrations stem from the exerting force of the expanding fuel/air mixture upon the pistons during the compression stroke, which in turn is transfered to the connecting rods. This force being exerted on the connecting rods, causes them to flex, and therefore vibrate BIG TIME ( connecting rod "harmonics"). Those continual harmonic vibrations travel through the connecting rods, and into the crankshaft,and the crankshaft acts as a conductor of those vibrations. And w/out adequate dampening of this vibration via a vibration dampener attached to the crankshaft snout, the vibrations will not stop at the cranshaft, but will travel into other areas, and engine components will be destroyed, and eventually the engine will fail.
As Mr. Reinhart has pointed out, some people have neglected to replace their vibration dampeners after they've removed them during the under drive pulley installation, and running any internal combustion engine in a street car w/out the use of a vibration dampener is an accident just waiting to happen. It's only a matter of time before the normally occuring vibrations produced by any engine will begin to destroy various engine components IF they aren't dampened, and it usually will not take very much time for that to happen either. But again, that has nothing to do with engine balancing. These are two separate issues (vibration dampening, & engine balancing).
Greater care in precise engine balancing is taken during hi-perf engine assembly so that the reciprocting mass will run smoother, and will create less resistence to the force of internal combustion. That translates into more power. It's the same reason why serious drag race engines are built with lightweight aluminum connecting rods, and lightweight pistons. Less resistence= more power. But that hasn't anything to do with vibration dampening.