you don't need any attachment for great steaks, but the best one for the job overall is probably the slow'n'sear
the vortex gives you a crazy hotspot big enough to sear one steak (or two small ones) - you won't get grill marks because the radiant heat will be usually be higher than the heat of the cooking grate.
Plus you're eventually going to ruin your grate (and probably your lid)
cast iron is great for getting a fabulous crust, but there's no grilled flavor. You can add wood flavor by adding chips or chunks to your coals - but you don't get that GRILLED flavor because the fat stays in the pan and doesn't drip into the coals. You can resolve this by adding beef fat directly into your coals though.
http://weberkettleclub.com/blog/2016/02/01/grilling-tip-smoking-with-beef-fat-and-onions/gourmet grate is ok, the marks look nice on big steaks but look rather silly on smaller cuts. Space is very limited.
grillgrates give overly dark, obnoxious marks. BLACK and gross, not what you want - while leaving the rest of the meat crustless and flavorless because, once again, no fat dripping into the coals.
Slow'n'sear gives you a hotter than normal zone big enough to sear several cuts. Use it at the peak of the heat for a super crust, or sear right as the heat dip begins for grill marks.
Either way, close the lid and keep the flames away. Flame bathed steaks taste like shit.