Thats fair enough,i just think other factors may have come into play there like you said with tuning the second time around,i can see a few hp from cats alone but to say 30rwhp is a big big stretch,look at project vf from street quick,they only gained 25 rwhp from a full exhaust,difillipo extractors,3 inch 100 cell cats and twin 3 inch exhaust,that was an already tuned vf ss with a bog stock exhaust with almost identical factory cats,those guys have an excellent track record,repeatable reliable dyno etc.
So to be saying that the factory cats are so restrictive that you can make 30rwhp once they are replaced with high flow cats and retuned is not correct and gives others the totally wrong idea,not sure whats going on with your tuner but the fact he believed the cats gave 30 hp with a tune tells me something is off.
Comparing dynos is a stupid thing to do and something that i absoultely hate to do but its the only way i can explain this situation.
Seriously if the cats were that restrictive and holding back 30hp a full exhaust would be netting close to 50hp with a tune after its already had a mafless otr tune,thats crazy talk.
The most obvious scenario is that maybe he did an average job tuning it the first time and actually did a decent tune the second time around,or the cats were fkd in the first place.