This backpressur issue always starts a huge debate and I have never seen a totally satisfactory answer but will be glad to give you my understanding of the problem.
It is a complex problem because we expect our engines to work over a wide range of rpm's and temperatures, both of which dramatically impact the exhaust. For best power, minimum backpressure is best, but at the same time, exhaust flow velocity shoud be maximim. Now, these are two things that can't exist together and therefore a balance has to be reached. Generally, more power can be found by opening up the exhaust but often at the expense of low end torque. This is becuse opening up the exhaust will lower the exhaust gas velocity which is vitally important in helping the exhaust gas to be removed from the cylinder, and to help with the intake charge comming in as well depending on your cam overlap.
Think of it as water flow in a hose. If you have a garden hose flowing 5 gallons a minute you have good pressure, but it is hard to get more water out. If you swithch to a 5 ft culvert, the 5 gallons a minute is just a dribble, no pressure, but also no velocity.
Your exhaust system needs to be balanced to your driving. If you don't rev the engine, or have small displacement, smaller exhaust is better. If you have a big block at the drag strip and live in the 6000 rpm range you want big open exhaust. Most hot rodders find power gains by going to slightly bigger exhaust, which will help support more power (bigger cam, more air, more gas, etc.)
If you have lost all your low end torque you probably need the put the exhaust pipes back on to get your exhaust flow back up. But I'll bet it sings at high rpms.
Related, have you changed your timing? When I had open headers (installed, drove to muffler shop) my timing was way off and didn't have a bit of power.
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