I don’t know if I would categorize my D20’s ACT scores as “slightly lower” as she has a 34 composite (which is within Duke’s 33-35 middle 50%) Lol.
But to answer OPs question, I would submit an SAT score if it’s at least very high 1400 (say 1480 or above) with the goal to try and get within the middle 50% range of accepted students historically.
I personally think that students that don’t submit a score ARE at a disadvantage because there will be thousands of applicants who took these tests early in their Junior year and will have high tests scores before the testing agencies shutdown in March. I would prep like crazy and take the SAT late summer early fall.
Also, if you are not getting the practice score you want on the SAT, maybe the ACT will be a better test for you. Most students do better on one test over the other.
If you can afford it financially and you know Duke is your #1 college, I would consider applying Early Decision. Last year, the ED acceptance difference was 21% vs 6% regular decision. The overall combined acceptance rate was 7.7%.
Even factoring in a “hooked” applicants for ED the acceptance rate was considerably higher applying early. And even if you don’t get in ED, 10% of the applicants were deferred and of those, 10% ultimately received an acceptance in RD, higher than the general RD applicants.
Lastly, I’m a big believer in “packaging” yourself. Tell a compelling story of why you would be an asset to Duke and your essays and LORs are going to corroborate the story. You want the adcoms to be an advocate for you in the committee meetings so give them a lot of ammunition!