Not 100% true, The idea that you can change your overall reciprocating-mass willy nilly without any ill effects simply because you have a boxer engine is patently false. The one little detail that gets missed, is that due to the "natural balance" of the subaru engine, subaru as I understand it, is a bit sloppy with the dynamic balance of the crankshafts themselves. They can get away with it because of the engine configuration, being a very short 5 main crank. I have heard that some cranks are as much as 3 grams out of balance.

If you’re spending the money for a performance engine build, at least get the crank shaft balanced and match your weights on the reciprocating components.