There is always going to be a processing delay associated with software audio filtering. If you're using software monitoring (as in, listening to the output of the audio channel in "real time"), then you will definitely notice the delay as it has to go through the entire audio chain of A->D conversion, software processing, then D->A again. However, if you're just talking about the recording/streaming aspect of any delay added, that can be compensated for by adjusting the sync in the Advanced Audio Options (click the cogwheel in the Mixer section).
As far as the question of whether it makes the dbx "useless"... that is 100% subjective. The compression method is always going to be different depending on what you use, whether it's dedicated hardware, or whatever software algorithm. The only way to really judge whether it's comparable is by trying it out for yourself, or listening to examples of each (preferably using similar settings).
Also, I'll just point out that you're not limited to the built-in compression filter either. You can use VST plugins, of which there is an entire multitude of different audio filters out there, some free, some not.... which
includes compressors.
