Somebody over on brianenos chrono'd loads with small and large primers. They lost 25-40 FPS with the SP cases. Kinda tells me you can use the same powder charge but the velocity will be different, and the SP cases should develop lower pressure for a given powder charge.
Target loads I wouldn't worry about. If you have a max load already then yes, reduce the charge and work back up.
FWIW and YMMV, small primer cases give me better accuracy when bench testing loads. For USPSA ranges you're splitting hairs, but 50 yard bullseye may be another matter.