Continuing with my research on this topic, I found the following conversation from the Mixxx Community Forums to be interesting:
https://www.mixxx.org/forums/viewtopic.php?f=1&t=460
This seems to indicate that there is NO added latency, which would indicate that JACK itself is at least equal to ALSA latency-wise. I suppose it is still possible for badly behaving apps in the JACK chain to cause latency. And it is also possible, I suppose, that when an app is tuned better for a particular technology (ie ALSA vs JACK) than another, then things could run better on one vs the other. That clears up a lot! So, it comes down to this:
Does "REAPER" run with lower latency and less Xruns when using straight ALSA, or through JACK when tested on the same REAPER project?