Before giving up on it, I would pull the timing cover and install/degree the cam straight up. Many chain sets have built-in advance anyway to compensate for chain stretch.
I know you are just relaying your experience, but the LT4 cam and 1.6:1 rockers should have given you a very solid, noticable 20-30 HP (depending on the rest of your setup).
EDIT...Forgot to mention had a sharp mileage drop-off and a tip-in hesitation start about 6 months ago on the Express. The city mileage went to hell and the highway mileage was down too. When you cracked the throttle in-town it would fall on its face, felt very lean. On the highway at highway speeds when you opened the throttle it still felt sick. Cold startups we often very difficult with random stalling immediately after. After checking the fuel pressure, reading everything with a scanner, I noticed that the EGR commanded % and actual % where nowhere close to matching up. So I removed the valve and found that the pintle was binding. The system never set a trouble code, but I replaced the valve and it was back to its normal 14-15 in-town and 19+ mpg highway. So don't overlook the small things.