Typically I believe these topics are viewed from the wrong end, and answered in the same manner. It's not really so much a question of what rod at all. That's really the last question to answer. No two lines or rods are created equal; even in relation to length. Also no two people cast the same, or like the same "feel"in a rod tapper. That being said; as you can see by now it's like asking everyone "whats the best car to drive"? You get 10 different answers from 10 different folks.
Smallies are my favorite query, and I guide a lot of clients for smallies in western PA. Therefore I get to cast a lot of set-ups. There is no one set-up to rule them all, but there's some good places to start. Lets start with what is truly important casting! It doesn't matter what gear you own; you got to bring your cast to the water. "Trout purest" love to dog smallmouth fishing, and I can't count the number I've met that can't hold a loop over 40'! Smallies require more period, so lets start with whats really the question here; What helps us cast smallmouth flies? Learn and practice a good single haul, and then a double haul. This will help you cast any wind resistant or heavy weight fly easier with any set-up as well as deal with the wind. Second:LINE: In all depth ranges I recommend using a heavy front taper line (like Rio cold water Clouser as an example). These tappers make ease of heavy flies and load a rod quickly which equates to less false casting, and less time for error. Thirdly; LEADERS: Fish will be at different depths depending on conditions and time of day, so we need to get a presentation to the fish without spooking them either. Therefore use a leader for the situation. I suggest tying your own. It's cheaper, easy, and more effective. The issue here is turning over a fly without your loop collapsing. When fishing a 2' or shallower target range I recommend a 9'-10' mono leader starting with 3'-4' of 50lb; 2'/40lb; 1 1/2'/30lb; 1'/20lb; 1'/15lb; and 2'/8lb-3X tippet. This will turn over any fly with a good cast behind it, and is long enough not to spook fish on those low clear summer days. For a 3'-5' target range I recommend a intermediate line with similar taper, and I do shorten my leader recipe to around 6' of Fluro. Anything deeper I like the newer steamer lines with a floating running line, and 15'-30' of full sink. These come in a lot of options these days, and again I shorten my leader to about 4' of Fluro. Usually 20lb-15lb-8lb tippet.
In my opinion the last part of the equation is the ROD. I like a fast action rod for dealing with these sort of flies, but that's my preference. Others may hate that feel; but the rod is merely a tool; so choose one that best suits the job. 9'; 9'6"; 8'4" it doesn't matter. The cast-ability matters. My Echo "84" 6wt cast just as well and as far as my Sage 596 Zaxis; they're just different. I do think an 8wt is over kill unless you're catching 3lb+ fish all day, but something LINED correctly in the 5-7wt range will handle the work load and keep the catching fun. Again for example My Sage 5wt 9'6" Z Axis will cast anything my 6-8wt rods will with the right line/leader combo. Rio Gold will not throw a steamer as well as an outbound short with the same rod. It's night and day! Again it all really comes back to your casting. $900 rods don't catch fish better than $150 rods, but your ability with it in hand is what matters. Well.... I know it's a bit of info, but it comes from a lot of years of experience; time on the water; and trial and error. I hope it helps, and feel free to PM me with any more specific questions!