It's fairly common practice to use a basic downstream 2nd as an octo with a balanced 2nd as your primary. The "old school" thought on this is that if you ever have a partial HP seat failure, which means the intermediate pressure rises quickly out of range, the downstream 2nd will start to freeflow more quickly than the balanced 2nd. The theoretical advantage of this is that you might avoid a sudden freeflow in both your LP inflator and the primary 2nd in your mouth, making a safe ascent easier. Basically the downstream 2nd acts as an over pressure relief valve. If you were to experience sudden total seat failure, which is very rare, all your LP connections would be subjected to HP air, and that's ALOT of air.
This works because the downstream 2nd is held closed only by spring pressure, so when the IP rises, the reg opens quickly. A balanced 2nd is held closed by a combination of air pressure from the balance chamber and a spring. So, when the IP jumps, the air pressure in the balance chamber also rises, and the reg won't freeflow as quickly. If the reg was 100% balanced, meaning the pressure in the balance chamber was equal to the downstream air pressure, the you could theoretically jump the IP really high and the reg would still not freeflow. Balanced 2nds are designed so that the pressure in the balance chamber acting to counteract the downstream air pressure is always a little lower, so the IP spike would eventually open the reg, before the LP hose would burst.
None of this has too much practical meaning these days, because sudden HP seat failure is really rare. And, I'm not sure how reliably the whole idea works; IOW in a seat failure situation, does having a downstream unbalanced octo really keep your LP inflator from suddenly and uncontrollably filling your BC, sending you to the surface in a big hurry. I bet in some situations it would work, but in other more severe seat failures you'd have both 2nds and the inflator on full blast. Not a pleasant situation, but at least you'd have some excitement on that dive! Thinking about that, one of those flanged fittings that installs over the LP inflator connection and allows much easier quick disconnect under pressure sounds like a good investment.
About the idea of having an octo that's as high performance as your primary, personally I don't think that's too important. What you need in that case is a reliable 2nd stage that breathes acceptably well, and there are MANY good basic downstream 2nds that fit this bill. One is the SP R190. The thing to avoid IMO is getting a cheap "compact" octo that breathes badly and is more likely to cause problems between services.
