Let's look at it from a different side assuming 12 energy/sec (ascension plus raid buffs).
Over the course of 88 seconds a Chi-X monk (with modest haste) will be able to generate about 5 additional chi that is not used up on 11x3-chi explosions and three guards. This gives a grand total of 16 purifies if the person wants to end at zero shuffle accumulated. Plus this assumes perfect execution and 100% uptime on the boss.
During the same period of time with serenity and with the same haste only ~30ish seconds are not covered and we'll round it to 80 seconds of traditional chi generation. 80 into 8 rotations each giving 4 chi, 5 additional BoK thrown in giving a surplus of 22 minus three guards leaving 16 left to purify. Note that this does not include the free purifies during serenity.
In the end BOTH are mostly even with about 16 purifies every ~90 seconds.
Leaving out other differences, there is one main reason serenity pulls ahead and that is what happens after a big hit or in critical health situations. To ensure that all tools are available as needed, most monks will keep 2 chi as a buffer to use guard instantly and followed shortly by purify in clutch situations. Chi-X dumps everything leaving you wide open. A Chi-X monk could use brew, but that is only available every minute not to mention requiring quite a bit of haste.