Spey is a style of casting, single handed or double handed rod are all effective by using spey cast to deliver the fly. I did some spey casting ,experimentally, from 3wt 6'6 to 7wt in some tight vegetated creek, and it did work great.
There are two major types of spey casting, it was classified depends on the "anchor” types. 1. Traditional spey: the anchor is "kissing the water", is type of touch and go, minimized the stick of the water tension.(e.g. Single spey and snake roll cast) Also, the whole motion of the cast is continuously exercised. Keep the line tension from the start "lift" to the "fire out". (Rio Grand Spey is the line suitable for this type of cast) 2. Skagit style spey. This type of spey use "water borne anchor", use the maximum water sticky tension to load the rod. The sequencial motion of this cast can be break down to several parts. (e.g. Double spey, snap T) ( I was very very impressed by Al Buhr's Skagit casts). It also very easy to cast a heavy shooting head, use several “broken down” motions to bring up the heavy head and also "increase" the water tension to load the rod and fire out. I start to use this type of casting not long ago and really love it for casting heavy flies. I think skagit cast have it's advantage in shooting streamers. The reason I say that is, you shoot out a long running line, and work your fly all the way back, then use the heavy head to shot them out again. Almost no false cast involve, it works very efficient to me. (Skagit line, heavy and short shooting head work for this type of cast)
So, I guess my answer is spey can do it all...but... For big water, I use double handed rod, combined with different lines. it save energy. For small water, I use single handed rod, combine spey and overhead casts. Depends on the creek condition. (carry a 13' stick around in a creek wasn't fun...) This is just my opinions...for your reference. (yuhina's response in WFF.com)
I am a biologist grew up with a fishing rod in my hands. I have been fortunate enough to embrace my love of art and science in my career. Most of my free time were well spent in the nature, either studying bird behavior or chasing variety of fish with fly rods. My favorite art medium is watercolors, I love the natural and spontaneous feel created by watercolor pigments. I am currently a post-doctoral research fellow at Harvard University. Welcome to my website. Mark