The codec performance for the Gigabeat S is misleading right now. Codecs aren't optimized for the unique advantages the CPU has, and the CPU is not being run at full speed. Please, be careful with repeating information about early ports.
As far as video goes, though, the Gigabeat F will run basically any video you're likely to throw at it that Rockbox supports at full speed, extra CPU speed beyond that doesn't gain you too much except, in some cases, battery life.