Just look at the S/N on your mic :-O .

The miniscule amount of additional noise incurred by running an ADC at -20dBfs is literally dwarfed by the mic, preamp, compressor & acoustic noise floor in the chain.  I regularly track in that range, but tend stay closer to -12dBfs.  Keeps the mixes nice and open IMO, and leaves some wiggle room for plug-ins w/o getting all anal about gain staging at every juncture (not that being anal about gain structure is a bad thing, but if you leave some headroom it is easier to manage gain structure in general IMO).  Tracking at -3dBfs will basically FORCE you to be anal about gain structure at every step with no s/n benefits to speak of...

Regardless, the RME AD/DA's have selectable reference levels (you can vary what 0dBfs is defined as) - so you can still run the analog gear in its "Sweet Spot" (varies depending on the gear in question) and still track at sensible levels.  So the correct answer is to track at levels you are comfortable with (gain structure management in your DAW), and where the analog gear sounds its best (how hard is the analog unit's output being pushed and where does it sound best?)