I think the smoke ring stops developing at or around that temp.
I put about 4 fist/baseball size chunks throughout my charcoal on any given cook so there should always be a burning chunk.
As far as adding wood, go ahead, but you risk too much smoke.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 10 WSMs (B, C, DH, DA, EZ, DZ, EO, DU, DR, EE/DU mutt); Brinkman Cimmaron; Chuckwagon Cooker; 5 22.5" Bar-B-Q-Kettles; 1 18.5" Bar-B-Q-Kettle; 3 WGAs (EZ, DE, N); 2 SJPs ( DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU).
"For when democracy becomes tyranny, those of us with rifles still get to vote"