[Draff1]

Based on a good deal of research with Dinan, Challenge, Gintani and this forum the consensus is that a full Dyno proven ~30HP is attainable with the right parts and tune. Here is my estimate:
Removal or primary 400CPI cats = 15
Removal of secondary 200CPI cats = 5
Tune = 8
Rear section = 3

While some of these parts may claim individual HP gains higher than the assigned value above, most people know that bolt-on parts are not exactly a sum game. Most times you look at added exhaust flow providing a maximum amount of HP/TQ before reaching diminishing returns.

In general, you can delete any one of these and expect to see dyno proven gains commensurate with the value I estimated. Clearly, the primary cat delete and tune are your best investment objectively. Maybe the worst power to dollar investment is the rear section but it dictates sound.

Consider the physical order of the restriction in the exhaust. Eliminating only the secondary cats when there is a far more restrictive cat in front of it will yield limited gains. If you do change your x-pipe your sound will change dramatically too. A tune is needed with many options simply to disable the CEL.