It depends on the car, and the size of the piping, etc.
For instance, a short-wheelbase car will notice less drop in HP between cat-back and axle-back because the exhaust piping between the cat and axle is shorter. Long-wheelbase cars will therefore see larger gains from cat-back.
In any case, cat-back is always going to result in more HP unless the piping is too small or too big. Tirod is right, 2.25" is perfect for a naturally aspirated car. Also, like he said, make sure the piping is mandrel bent.
I don't have any charts handy, but I bet you'd get about 50-75% of the gains of a catback from an axle-back. In other words, if you made 10 hp from a catback, you'd make 5-7.5 hp from an axle-back. That might be optomistic, though. I wonder if anyone has any good research on this....