
Balance shafts are "required" regardless of how many counterweights are on the crankshaft. They are quelling second order vibrations (unrelated to the first order vibrations you're talking about). You can have the perfect inline 4 cylinder crankshaft and you will still experience the same magnitude of this specific vibration mode when compared to a lackluster crankshaft in the same engine. They "need" the balance shaft (versus an SR20) because of the stroke length, and the weight of the piston/rod combo.