Backpressure itself will not help fuel efficiency at all. Adding an exhaust that makes some backpressure under some operating conditions (mostly talking about how much is flowing through and how fast) can easily help or hurt FE, depending on what range of flow is helped or hurt by the exhaust.
Clear as mud, right?
You're not actually going to
know until you try it out, and the results from fitting one exhaust may be fairly different from those for fitting a different exhaust.
A catalyst cannot really help your FE. One that's broken up and partly clogged can certainly hurt flow and FE.
The location of the muffler can affect efficiency, as it affects the other components in the exhaust. For instance, it could give you less room for long primary pipes (or long secondaries) if you put it too close to the engine. Or you could put it somewhere that needs lots of tight bends in the exhaust pipes to reach. I doubt that putting it in the middle of the vehicle (which seems to be what you are implying) or the tail end will affect things noticeably, though.
Exhaust design is a mix between art and science. You can build things that are optimized for a given RPM, or if you're lucky a decent range of RPM. In general, long narrow pipes are better for slower flow (i.e., low RPMs) and short fat pipes are better for faster flow (i.e., high revs). You can also tune the lengths of the primary and secondary pipes to reflect the exhaust pulses back as mini-"vacuums" which will help flow. And of course, all of the bends affect how the gases flow.
And there are exceptions to pretty much every rule, because what fun would fluid dynamics be if they were actually easy to calculate?
-soD