I'm still trying to learn more about balancing before I make any more decisions about it, but finding information specific to a twin cylinder is proving tricky!
Unfortunately this is one aspect of tuning that I am intrigued about and your postings have drawn me into wasting many hours scouring the internet in the same "vein ".
I found an extended German discussion about the BIS crank and the relative balance weights and ratios for vertical and horizontal engines. It does seem that the heavier crank in combination with significant lightening holes in the flywheel is necessary in the BIS, mainly in response to the lack of gravity assistance on the downward stroke of the piston.
If I read it correctly they were saying that the vertical engine responds better to having a proportionally lower percentage of the piston and conrod mass built into the crankshaft counterweight. If the BIS crankshaft counterweight is heavier than the standard version, this implies that it would need significant machining before being safe to use in a standard-spec engine or in one with lightweight internal components.
It is only fair to say, (as did one of the German commentators), that my thoughts are based only on cramming myself with online information gleaned over the past few days.