UPDATE:
This past Friday night, I played the same outdoor gig that I mentioned above using the same emergency rig once again feeding the PA.
A friend of mine was in the audience and took this bad cell phone pic of the band on stage. The QSC monitor sat just outside of the picture frame to my right.
The stage actually extends another 10-15 feet in either direction beyond where we set up - so it's about 30 feet wide by at least 15 feet deep, which is a little larger than I originally thought. We placed the PA speakers near the far edges of the stage.
This time, I pushed the QSC to 2/3 of full volume and it helped to cover the stage area a little better. My friend, who is also a musician, said that the bass sounded great and cut through the mix nicely, sounding full, thick and defined.
Given this review and the fact that I can carry everything in one trip, I'm strongly considering changing the name of this rig from emergency to permanent...