Ok, 6500 RPM it is... Long time since I tried it...
Really, there's no point in hitting above that, considering the peak hp is at 5400 RPM and peak torque is at 4400 RPM.
I *think* a malfunctioning EGR system can also cause very poor mileage, but usually would come up with a code.
Have you tried disconnecting the battery overnight to force the ECU to relearn? If it's trained on sub-par gas, it may have an effect.
Throttle body cleaning shouldn't cost much, since it's not that hard to get to. I'd guess an hour's labour or so, but I really don't know the book rate, since I did it myself.
The torque converter lockup should be smooth, similar to a 3rd to 4th gear change. If it's "slamming", I doubt that's good.
VK