You will get a check engine light for catalytic converter below efficiency even with high-flow cats installed. However it won't put the engine in danger by itself. What I would pay very close attention to is the Oxygen sensor wiring harness. DO NOT USE EXTENSIONS
. If your headers come with extensions, throw them away. The factory harness can be extended to fit the new headers' o2 sensor location by removing a tie-down on the upper bolt of the trans
I would take a canned tune from an experienced 5.0 tuner over a custom dyno tune from a local shop any day. The shop that dyno'd my car did not even know the 1:1 gear ratio. I am lucky to have some of the top shops with a few hour drive but not everyone can say that.