***all data from nba.com stats, bbref, and nbawowy as of Dec 14 morning***
Several people here have praised Speights' greatly increased rebound rate this season.  Notably his def reb% is now 25.8% compared to 19.8% last season with us.
I have watched many Clippers' games this season, and he did not impress me as "controlling the defensive board".  And the Clippers didn't look that great on the defensive glass when he was on the court.  So I wondered why, and started looking at contested rebounds and his on-court team dreb%.

1) His dreb% is 25.8% this season, actually +30% better than last season
2) His contested dreb/min is slightly better than last season at +10%
3) His un-contested (empty) dreb/min is +21% better than last season.
4) His team still def rebound badly with him on the court... 73.6%... similar as last season.  73.6% dreb% is actually *worse* than the #30 dreb team this season - the Knicks at 74.0%
5) His contested rebound/min is actually *lower* than David West - who many here believe is a worse rebounder than Mo
6) West's on-court team dreb% is 77.4%, *way* ahead of Mo when both play at center.
My conclusion is Mo's rebounding prowess is not as good as his 25.8% dreb shows.  He got many more empty rebounds this season because he plays a lot with Crawford, Felton, and Rivers (low dreb% players), compared to Livingston/Iguodala/Barbosa/Barnes.
It is true that he is getting 10% more contested def rebounds per min, which is a positive thing.  But his team still gets pounded on the offensive glass like worse than the worst def rebounding team when he is at center.
Mo's defensive rebounding prowess and his improvement in it is overrated.  Myth busted.