How much horsepower does a 1997 chevrolet silverado 1500 Z71 with a
5.7 liter engine gain from removing the stock catalytic converters
with flowmaster true dual exhaust

I was going to answer your question and then I realized that
everything I had to say was negative. I could find no proof of
increased horsepower and most sources say that because of the O2
sensors and computer settings you could actually lose efficency. I
talked with a tech guy at Flowmaster and he had nothing good to say
either. I would be glad to give you an answer, but I don't think you
would be satisfied.

Well I know for a fact it increases horsepower.  I didnt pay $20 for
that and I am not going to allow payment if that is all I get.  Just
in case there was a misunderstanding.
I had stock cats and then put true dual exhaust by flowmaster on my
truck.  I then took the stock cats off and had straight flowmasters. 
I know it has increased my horsepower because my truck is much faster
than it was before.  I CAN TELL!  I know that truck inside and out
which is why I want to know how many horsepower I haved total.  Keep
looking buddy if you want the dough.  Thanks!