Don't base your axle back decision on power, because the difference will be like 1-2hp so go with what sounds the best. Check out my S197 Exhaust sound thread unless you have slow internet. If you do, I'll let you know when it's safe.
Long tubes and an X will in conjunction with a good tune and CAI be responsible for roughly 20-30 rhwp gains. As stated above, axle backs don't add anything worth measuring. Maybe 3 hp if you're lucky. Choose those based on sound as RF said.