I can now confirm that after running it for 1 1/2 hours without a recharge, this light, the Ultrafire WF-501B, is still considerably brighter than a PBSF with fresh batteries on solid mode.

I won't call this tail light a direct replacement for a PBSF, because the PBSF has better side visibility. But they work together well.

If I were to do it all over again I'd order this tail light with a P7 and a PBSF to be run in blink mode.