For switch rods, especially for overhead casting I would stick to med-fast or fast actions rods. The med fast to faster action rods definately have the advantage when overhead casting.
The slower more med or traditional action switch rod are to slow and loopy for overhead work in my opinion. These rod would be the Cabela's single hand spey rods and the Wild Water Rods. They are good rod for roll and spey, but should be avoided for overhead work. IMHO
--------------------------------------------
My recomendation of rods to look at for overhead spey and truly versitile use would be
1. St Croix Imperial Switch, around $250
-I have one and love it, fast action and really loads easily with two hands for overhead cast. Well worth the money.
2 Echo SR Switch rods, around $320
Great rod a little udifferent on the color sceme look, but make up for it in the fast, easy use action on these rods.
Their are others company like the TFO Deer Creek Rods, Reddington, Beulah, and countless others out their but the two listed about are the lower priced end that others seem to say overhead well.
-----------------------------------
I know you originally were thinking 8wt, but something to think about.
With a switch rod because of the different in lines and lines weights used on a 8 wt switch is more like a 9 or 10 wt rod in line size or weight.
For example my 6wt switch for overhead throws a 300 grain head, that weigh is almost the same as a 10 single hander.
You may find depending on what you target is that a 6-7 wt may be closer to what you need.
Switch rod are just plain fun and a new kind of rod to add to our arsenal and require a little learning curve to full utilitize them.
hope this helps
jcspey