this is another new member looking for guesses ..
your checking on your mechanics work .
The forums cannot help you unless you post important troubleshooting results.
engine has NO power so he does a tune up and nothing improves NOT surprised on that one.
answers required are what is the fuel pressure. also does the fuel pressure hold key on engine off. does the fuel pump deliver good volume this is checked with the fuel pressure gauge connected that has a drain valve open , see how the pressure holds .. dirty injectors POOR flow.
downshifting on an engine that has limited power will not shift properly . the engine when at WOT should roar not whine like a little BEACHe..