I wouldnt be surprised to see results like that at all. The cam in a 5.3 is really small. This cam is fairly similar to the lingenfelter GT2-3 and that cam shows some crazy gains on the 5.3's sometimes. Did you get it tuned along with this cam change? The 2 combined I would expect more than 40 HP to be honest.

 

I did the LS6 cam in my Avalanche with a LS1 engine, it made the high end of the RPM range much better but its a dog up to about 3200 on the low end. LS6 would probably be ok in a lighter truck.