Fly fishing is unlike spin/cast fishing in that you can always cast
smaller flies with heavier fly tackle (e.g., 8-weight) whereas spinning gear
sucks at casting lures below a certain weight. What you are capable of casting will largely depend upon your casting ability, type of fly line (lines with shorter front tapers make it easier to cast heavier stuff), wind, wind resistance of the fly itself, leader, etc. That said, it is certainly easier to cast flies with lead eyes (and larger flies) with heavier fly lines. An 8-weight will comfortably cast flies that are larger than what I generally prefer to cast, period. Fortunately, it's also a great line for handling wind and getting good distance though -- so it's absolutely a good choice no matter what you do around here. We've caught snakehead, carp, catfish, and some fairly nice bass (probably up to 4 pounds) on flies tied to a size #10 hook.
Fly rods are good at presenting and manipulating small to medium-sized lures in shallow to moderately deep water. That's where you get a real advantage to using fly tackle in my opinion, and it's the most fun kind of fishing there is! ;)
I don't have any interest in trying to force a fly rod to be something else.
With any 8-weight you should be able to cast size #6 up to #2 (or possibly 1/0, I don't know exact hook sizes at the big end...I have some even bigger stuff tied on some old eagle claws that I will never use) lead clousers and #4 blue poppers which are probably the most common flies people fish on here.
Gene