Hey Matthew, missed you at DLR a few weeks ago! I did meet some other DIS folks.
Wednesday is the best day for DL. On Mon, Tue, Thurs and Sat DL has EE, which means people with EE tix get in an hour early. Thus there are lines already in FantasyLand and TomorrowLand, even if you get there at regular park opening. On Wed, there are no DL lines at park opening.
We were at DCA for parts of all four days of our Aug 1-4 trip. Our experience was not the same as Matthew's. E.g., we rode Grizzly River Run on Thursday morning and the standby line was pretty long (at least 40 minutes). On Tuesday at noon it was 70 minutes. We used FastPass both days on GRR, which meant a five minute wait. Standby lines for ToT never seemed to be longer than 30 minutes. We used FP twice for ToT. In summary, I do not think any non-weekend day of the week is better than any other at DCA.