i always ask on these topics...why would the drop rate be upgraded? you have 14 days to finish 100 pieces...that's about 7 per day and i just dropped 5 in 5x15s jobs today, in a row.
and as a side perspective:
i don't think a cap can be put on quest objects because there are also the ones sold in shop...and without the quest objects added to the drop...they are kinda very profitable. never paying for premium ever again profitable
maybe as an alternative just keep the unlimited drops for the ones in shop and capped for the ones on drops...