We shot round 2 yesterday, and I went back to the 50 to try again. This time, I shot in "AI Focus" mode instead of "AI Servo" mode, and the 50 worked great. This left me feeling a bit unsatisfied, however, because I couldn't really explain why I was seeing the results I was seeing. The user manual, for instance, implies that AI Servo should be ideal for moving subjects.
@Itai's answer turned out not to be entirely correct, but it gave me exactly the clue I needed to figure out what went wrong in AI Servo mode. A little googling turned up a very helpful article with information that's purportedly straight from Canon. The key information begins around item 3 -- paragraph A indicates that the first shot in AI Servo mode uses Release Priority, and paragraph B indicates that subsequent continuous shots use Focus Priority. Although this information is given specifically for the EOS 1, circa 1992, I've yet to find anything indicating that this has changed, and given Canon's tendency to respect backwards behavioral compatibility, I don't expect that this behavior would change without the addition of a menu option to maintain the prior behavior. Custom Function III-1 seems to be close to to this sort of setting, but I don't think that's really the same thing as release priority vs. focus priority.
Further searching turned up an answer that seems to confirm this and extend understanding to encompass what I saw yesterday:
AI SERVO (at least in the iterations
before the 7D, i.e. 20D, 30D, 40D, and
perhaps 50D (dunno never had one))
will keep shooting at full speed as
long as its getting (what it thinks
is) focus lock for every shot. With
the 7D (and perhaps 50D) there are two
C.Fn's that handle how AI Servo AF
C. Fn III-2 "AI Servo 1st/2nd image
priority" 0: AF priority/Tracking
priority 1: AF priority/Drive speed
priority 2: Release/Drive speed
priority 3: Release/Tracking priority
of the 4 choices, Choice 2 is the
fastest because even with the first
shot of the sequence the AF system
does not wait for AF Lock, its
priority is shutter release.
of the 4 choices, I use 0 for the
simple reason that the 70-200 f/4L IS
USM which is the main reason for such
speed for me can actually keep up. 2nd
fastest lens is probably my 100mm
f/2.8 Macro USM, followed by the 17-55
f/2.8 IS USM. The speed of the lens's
AF motor plays a major role in how the
AI SERVO will work. If the lens is
slow and your priority is choice 0 for
example, you won't likely be able to
maintain full 8fps on the 7D. I know
on my 40D and 30D before it, slow
lenses like the 75-300 III would no go
faster than 3fps in AI SERVO, even in
High Speed Continuous motor drive.
Simply put AI SERVO was originally
designed to keep shooting as long as
AF is found - no lock, no shot.
What I used to do with my 40D before I
got the 70-200L was I'd set my AF to
ONE SHOT and High Speed. It ended up
doing the same as option C.Fn III-3
Option 1. It locks for the first shot,
then just shoots more of the same
focus. AI SERVO on the 7D is a lot
better tho because now it can still
give a damn about focusing the
subsequent shots - something ONE SHOT
AF is not meant to do.
C.Fn III-3 "AI Servo AF tracking
method" 0: Main focus point priority
1: Continuous AF track priority
This function deals more with tracking
priorities... i.e. if something gets
between you and the subject over the
course of a sequence... like a tree
that gets between you and your dog.
________ Alan "NuReality" Fronshtein
This answer supports the idea that the motor speed of the lens is part of what may have been slowing me down with AI Servo, since that mode depends on focus lock for continuous shooting on shots 2-n. It sounds like later Canon bodies, including the 7D, have an implementation of AI Servo that includes better control over the use of release priority vs. focus priority.