[ngoma]

Welcome to the forum!

Hard to tell from a distance the exact culprits of your lack of top end power.

I am guessing that you are perceiving a marked difference in performance and top end speed pre- and post- head gasket replacement work, correct? (Or did this just happen more recently? You mention it has worked perfectly for 10k miles, so maybe not?)

0-60 what should it be and what did you measure?

Injectors: given that they were not changed, we can rule them out as contributing to the difference in performance before/after.

Muffler appearance shouldn't matter unless it is clogged or incorrectly sized. Why was it changed for a HG R/R?

Turbo: Can you attach a boost gauge to help monitor what it is doing?

That amount of oil use is not bad.

Sudden loss of power (boost) under full load: Momentary or sustained? High RPMs, or low RPMs? Does it let out big puffs of smoke when this happens?

These are all questions to help us match up the symptoms to known culprits.

PS: These engines have suffered greatly at the hands of (otherwise good) mechanics not experienced with the specific idiosyncronicities of this engine. What about your mechanic and his/her track record with these engines?