2c's sweet spot for power is between 2500-3500 rpm and reving above 4000 doesn't yield anything except smoke.
Though I know a thing or two about diesel but I'm no expert. But owning experience of several 2.0Ds in past, it shouldn't feel lacking power at low rpm, in fact this engine is meant to be driven at low rpm all the time. during usual driving as long as you don't make it shudder below idle rpm it doesn't need a downshift.
Due to heavy clutch operation, I tend to skip a gear like from 1st to 3rd than 5th or from 2nd to 4th than 5th when shifting at 2500-3k rpm in each gear but it never feels like lacking power though my current one is in dire need of checkup because it is refusing to rev past 4k rpm even in neutral, like between 4-5k rpm it needs like 4-5 sec of accelerator floored even in neutral to hit the redline, but up till 4k it is a rocket.
get the usual maintenance like air filter and fuel filter changed and if it doesn't improve than get it checked according to the list in this blog post
and keep this thread updated.