You planned a certain TAS so it's reasonable to set the power you need to get that TAS, providing you still have sufficient fuel reserve and the engine is operating within limits - if not you need to adjust your ETAs and headings (stating the obvious).
It's rare for aircraft to achieve flight manual performance when they're middle-aged (- humans are the same :-) ), one that's only 6 years old should be better. Out of balance? T&S not correct in panel. Something out of trim?
HFD