The stock final drive ratio is 2.125 : 1 (68 / 32). If you substitute a 66 tooth rear sprocket, that would give you 2.062 : 1 (66 / 32), a 3% difference. And yes, the smaller rear sprocket would drop rpms approximately 3% at the same road speeds. Less acceleration, lower rpm cruising. The engine should be able to handle the slight extra load, a very discerning rider would probably be able to tell the difference in take-off from a stop and roll-on performance.
The trick is finding a sprocket that fits the 2010 in non-stock sizes. Do you have such a part? The 2008 FLHTCUSE3 had a 66 tooth rear, but it used a different style rubber damper than the 2009 and later models. I'm not sure if it could be used or not on the 2010. Btw, the reason Harley changed to a 68 tooth sprocket in 2009 was due to complaints about sluggish take-off and top gear acceleration if I remember correctly. The Cruise Drive trans ratio's weren't a great match for the 2.062 final drive ratio.
Jerry