Veloc - my take on this is this :
Last time I had my power band shifted up like you was when the flow of the exhaust was too 'smooth' or 'slow'. I had to add restrictions like a S-flow muffler to normalise it. So to me it sounds like you might have the same issue. 4AGE being a 1.6L, I'm thinking the 2.3" piping probably is too large. Like what most of the sifus said here, I think your issue is the large piping. I don't see a problem with adding the additional mid-box, but this is just my personal opinion.
What is the piping size from header right to the back? The mid-boxes are the same size as well or step up/down piping sizes?
Last time I had my power band shifted up like you was when the flow of the exhaust was too 'smooth' or 'slow'. I had to add restrictions like a S-flow muffler to normalise it. So to me it sounds like you might have the same issue. 4AGE being a 1.6L, I'm thinking the 2.3" piping probably is too large. Like what most of the sifus said here, I think your issue is the large piping. I don't see a problem with adding the additional mid-box, but this is just my personal opinion.
What is the piping size from header right to the back? The mid-boxes are the same size as well or step up/down piping sizes?