If the cast bullets are flat points, you'd be surprised at the amount of shock that can be delivered, even without the hollow point. Don't believe it, try filling a milk jug with water and having a go at it with the flat points.
I see no reason why you should take the op rod out to shoot cast bullets. If you work your way up, you'll probably find that the cast bullet loads can operate your SKS reliably. I read something somewhere about someone trying to foul a gas system with cast lead bullets. After a few thousand rounds, they finally gave up. Said there was "no noticable decrease in performance" due to any sort of lead fouling. I figure that you just need to clean fairly often.