MAN LE403 engines

Aha, I didn't understand that this is what you meant. Dark-ish smoke for a short while, at the moment when the engines are struggling a bit more to get the boat on the plane, is nothing to write home about, IMHO.
I would just try to use such "transition" speed as little as possible, because it's not efficient for the hull it and puts a high load on engines for no reason.
Either stick to displacement speed when you fancy going very slow (and/or sea conditions are bad enough to be problematic at planing speed), or bring the boat up to anywhere from the slower but steady planing speed (which I'd expect to be around 1500/1600rpm on your boat), and up to 2000 rpm (which you can keep for as long as you like).

Ref. the injectors check, that's a job normally NOT done directly by a MAN service, but rather from a Bosch service, like those dealing mostly with truck engines. I mean, yes, the MAN mechanic should pull the injectors and reinstall them, but that's the easiest part of the job. For their calibration and nozzle replacement, also MAN shops rely on a Bosch service.
So, that is the part where a competent mechanic matters the most!

Beware, you're mixing up different concepts.
The engine load is a percentage that can be anywhere from a single digit figure (with the engine idling and no gear engaged), all the way up to 100%.
You can only check that on the digital display that you should have in your dashboard.

The boat total displacement can and does affect the engines load throughout their speed range, but it tells nothing of the engines load number that Sarnico mentioned.
That's an apple vs. orange comparison, sort of.

Since you mentioned the Adriatic, in the northern (Italian) part I can safely recommend Centis Nautica.
Not cheap, but you get what you pay for...