Don't know anything about the magnitude of the benefits provided by high flow cats. I suggest that this path would be preferable to either of your other options listed, both of which are illegal.
However, not sure what the vehicle inspection or enforcement activity is like in your area.
Of the remaining two options, the test pipe gives you higher exposure than hollowing out since it would be picked up by a simple visual inspection. It may also present problems when you sell - may have to put things back right as a condition of sale. Muffler shops can get in serious trouble for installing test pipes for reasons other than testing.
Also, laws change. Should your area adopt mandatory emission testing, you'd have to put things back right anyway.