Don't confuse smoke and the smoke ring. The smoke ring only forms up to about 140 degrees meat temperature. However, smoke will continue to stick to the meat and add to the flavor profile throughout the cook. Therefore, there is benefit to continuing the smoke to the end of the cook....however, it seems logical (although I am no expert) that as the bark forms and dries out some, less smoke will stick to it.
A couple of things you may consider to help maintain your smoke throughout the cook:
--bury a few pieces of smoke wood in your unlit charcoal....these will light as the charcoal burns down through the ring
--use lump charcoal vice briquettes (I know I get more flavor from my WGC lump than my Stubbs briquettes).
I use lump exclusively for my long cooks and everything I do in my WSM. I use briquettes for my kettle (hamburgers, hot dogs, chicken, etc.) The WGC lump leaves extremely low ash and burns really clean. Yes I will get a couple of temp spikes with it and it is probably not as steady as briquettes, but it won't vary drastically as long as I break up the big pieces and pack everything nice and tight. Also, a ring of the WGC lump will burn forever so you shouldn't have to worry about reloading and you don't have to worry about the ash buildup.
Just some things to consider.
Kg