G'day Glen,
I reckon the exhaust is worth the money, I can't really say if it affects fuel consumption as I don't do enough long trip to get a good idea but it certainly helps with getting the boost on earlier, it frees the engine up a bit, acceleration is better, and the exhaust temps are lower which should lead to longer engine life.
With regards to size, go for 2.75", 3" is a bit tight, and I don't think there would be any benefit over the 3", if you want it quiet go with a straight through muffler, or if you don't mind a little noise go with no muffler.