I did as you suggested and sent my question of alternator speed vs. output to the Aeroelectric Mail List and received the following response from Bob Nuckols: (my estimate that 4,000 rpm would produce full-rated current wasn't far off....)
Here's a typical output current curve for an
automotive alternator. The population of
mfgrs and part numbers will "wiggle" a bit
from this curve but it's probably plus or minus
10 percent for the whole lot.
It depends on what you want . . . and what the tradeoffs
are. If you'd like to have good output at ramp idle and
taxi speeds (most folk really like the low speed
capability in both cars and airplanes), then leave the
pulley small. If you're worried about bearing and brush
life, know that B&C has been selling the L40/L60 series
machines for about 15 years with thousands of units in
service. The first wearout in B&C history arrived back
in Newton about a year ago . . . needed brushes . . .
after 2200 hours in service. These alternators run nicely
at over 10,000 rpm in cruise with no evidence of having
"over stressed" anything due to the high speeds.
If it were my airplane, I'd leave the small pulley
on . . . especially if your ring gear pulley is smaller
than most . . . you've already taken a hit on pre-flight
output capability.
Bob . . .