The big bore kit is more money than the hc pistons, but I would agree, in the long run, the big bore would be much less stressful to the rest of the engine and it's systems, requiring fewer additional repairs or modifications. This would make it overall a less costly way to get more power from a 912UL.

As an aside, in about 1995 the gearbox components were updated. The gears were widened and the oiling inside the gearbox flowed differently. This makes the later model gearboxes more robust, in my opinion.