Yes-Johns, like Aitken, is strong through the thighs and hips and can break tackles. On the other hand, Herbert is a great passer of the ball - he put Pereira over in the corner a couple of times against Wests with laser passes straight onto his chest. Herbert is only relatively short and slight in build so maybe 5/8 is his best position.
Speaking of shortness, it seems to be a backline problem with SGI apart from Nene. Aitken, Nightingale, Dufty, Mann and the ISP backs Herbert, Pereira, Tohi are all short in an era when outside backs are going tall, athletic and strongly built. Lomax is tall but a bit off a stringbean. The Warriors wingers are examples of that as is Melbourne's Vuliasu.