from the pictures it looks too lean, but pictures can be decieving. you said something about the midrange not being right. Is the midrnge rich or lean? if it is lean this will surely stick a piston faster than you think. something to ponder, when you let off the throttle, your no longer on the main jet (most common changed for jetting and percieved as a cure all) and you begin to run off the needle,needle jet,slide cutaway,and pilot jet. now think about how often you are on and off the throttle, it'll start to make since why mid range seizure is so common, but the mostly overlooked area.
This was one reason why I ended up Ghetto grinding a needle on Big Toms pilot for the 24hr race. The mid range egt went through the roof once you let off a bit and held it there for any duration of time.
when you put the engine back together, do a pressure test to double check the seals. it can be quite easy to nick a seal and not know it. Plus this will rule out one more possibility of what went wrong.