Ok, so what's the problem here?
Is it starting hard all the time or just hard starting hot?
If just hot, is there a heat shield on the starter? Have you verified that timing is not over advanced? Have you retrieved and Trouble codes?
Does it have the original vortec "intake spider"? Dropping mpg is one of the signs of troubles with the poppets. Hard starting can be another.
MPG is highly dependent on many things, even cold weather will throw it for a loop. It's very difficult to compare one tankful to another. What needs to be compared is a fuel log over many months. That minimizes variable such as long idle times and traffic, things that change all the time. A simple too many trips through the drive through can ruin mpg.
Also, HP doesn't equal mpg. That's a misconception many people have. Efficiency equals MPG. Sometimes (most times) HP comes with it, but that is a byproduct of efficiency. You can build an engine for hp and loose mpg, but you can't build one for efficiency and loose HP.
MPG can be a function of something as simple as changing differential gearing to bring the engine back into the torque peak with larger tires. That's chasing efficiency in the system, not just the engine....
