170-175 is great. Don't bitch - you're doing good.
As for lugging, the only thing to be aware of is that you don't want to run where there is no boost on a turbo engine but you're plowing. If the boat's out of the hole and on plane, its fine. Ditto if its settled (but that's slow!)
There is a "no-go-zone" for long term running which is where the boat is pushing a huge wave, stuck in the hole, and the engines are working their butts off but there's no boost. If you have Pyrometers you'll see sky-high EGTs - that's what you need to be worried about.
As the boost comes up the EGTs come down dramatically, then start rising again.
Prop demand curves are typically logrythmic, not linear. Its not a truck.