I've been kind of wondering the same thing for a while. Sure, for max. gain, you may want a cat-back, but if you can get 70% of the gain (that's just a wild guess) for 50% of the price, then mebbe just an axle-back would be good enough for those of us who aren't planning anything too major. You can get most of the look & sound of the others too. (And maybe less cabin sound...)
For around $250 or less, I've seen a couple of axle-back systems. One by Thermal Research looks kinda stock, with two 3" tips. Another, by 5Zigen, looks like the "N1" style, with the huge single tip.
But it's hard to get a straight answer on the performance gain on this vs. a cat-back.