congrats on your new kettle accessory.
Let us know how it works out...
I have the kettle, without your accessory, and weber genesis gas grill and I do have the rotisserie accessory for the gas. I have had the rotisserie for about 8 or 9 years, and only use it about 3 or 4 times a year, mainly for whole chickens, but the family prefers whole chickens cooked on the kettle. Before I had a smoker, besides doing a lot of smoking in the kettle, I also smoked pork butts using the rotisserie on the gas grill (be sure to truss up the butt with butcher's twine to keep it from falling apart). Rotisseries do give you more cooking options.
That sleeve can give you more capacity for smoking too, getting the meat further from the heat and thus more heat control. However it would be a pain to add more briquettes.