Not at all, Katiebear. A lot of our members develop their own lines of really stellar meat mutts, simply by selecting for the qualities they want in their herds.
I'm talking about the phenomenon of "hybrid vigor" seen when you take two completely different breeds and cross them. The litters from that pairing *should* show faster growth than a "purebred" litter. However, this isn't always the case- if you have two substandard animals to begin with, you aren't going to affect a miracle in the litter.
Supposedly, that hybrid vigor is lost once you continue breeding the cross, and the litters just grow at a normal rate.
I haven't tried this personally- but a lot of commercial breeders cross NZs to Cals for that superior growth, and since they (hopefully) chart the growth of their litters to determine profitability, I believe that there is some truth to the "hybrid vigor" theory.