Andrew,
I am not completely sure how to answer this as you did not state what Xerox printer you are using. Since not many printers have the Auto Toner Purge setting, I will assume that you are using either an iGen3 or iGen4 printer. If you are printing duplex on 12x18 stock, then the first speed that you quoted of 21.45 sheets per minute is not far from the theoretical maximum print speed of that size stock of 50 impressions per minute (25 duplexed sheets).
However, to answer you question, the purge setpoint changes the parameters we use for deciding when it is necessary to perform a Print Quality Adjustment (PQA) cycle in order to ensure acceptable print quality. A lower set point should cause more frequent PQA cycles as it indicates the desire for improved image quality at the expense of throughput. Of course, if you are printing something with fairly high toner concentrations of all toners, then no PQA should be necesary regardless of setpoint. Since your job utilizes no Yellow or Magenta toner however, the machine will, regardless of purge setpoint, go into PQA cycles with a run of this length. That being said, changing the setpoint back to it's original value will have no negative impact on your press other than the possibility of invoking more frequent PQA cycles that impact productivity.
