I thought about calculating the number of 15 minute publishers by watching the change in average hours. However, their impact on hours is not statistically significant.
I heard somewhere that tests of the 15 minute rule resulted in a 1.6% increase in publisher count (I can't remember the source). That's about 1 or 2 per congregation, which seems reasonable. A 1.6% increase in total publisher count, with virtually no corresponding increase in hours, results in lower hours per publisher of only 0.14 hours - a difference which is not even noticable, and cannot be distinguished from other fluctuations.
On the point of the reports being collected by the BS conductor (I like that phrase), in my congregation, the secretary always used to collect them himself. At least once last year my bogus 4 hours got missed. That seems unlikely under the new regime. Even a tiny increase in reporting accuracy will impact these figures.
The most accurate way to assess the impact of the changes is to wait until next year, and then compare equivalent year over year data.