Since you're asking for opinions, I'd say that upping the strength of a German lager which is meant to be delicate will unbalance the flavor. You can add dextrose a couple days into the primary ferment, or concentrate more to up the gravity. To best keep the balance you could do half of each, since a higher gravity ups the flavor, mouthfeel, and viscosity; adding a simple sugar like dextrose will do the opposite.
I really appreciate the munich dunkel style, so my true opinion would be don't do it. Stay with the original recipe. A good munich dunkel should be clean tasting with light mouthfeel, lager yeast snap, with sweet and slightly nutty/toasty flavor.