gth,
What is it you're after?
Spey casting because you think it'll be easier on your shoulders? If you're a long time fly fisher, casting a 9 1/2' 8 wt from daylight to dark shouldn't cause any more shoulder stress than you'll experience with the Spey rod, and especially so while you learn to Spey cast.
A Spey rod will extend your control a little further out on long casts. Although they can cast a country mile, I find I don't actually cast or fish more than about 10' further out than when I'm using my single hand rod.
What constitutes a waste of money to you? If you've already got a rod that casts and fishes well, any subsequent rod could be described as a waste of money. If you mean you don't want to buy a rod that you end up not using, then, well, there's always that risk when you try something new. Even if you try a Spey rod before buying it, until you get the hang of it and develop a personal preference, you won't know what to look for, in my opinion.
I don't know the Cabella's Spey rod, but I've heard it described by others as OK. A good value alternative to the high end rods is the blanks sold by Anglers Workshop. I've cast their 12 and 14' two handers, and have been using the 12' for 3 or 4 seasons now. These are a pretty good value if you like to build your own rods from blanks.
Good luck, and let the rod do the work while casting. Save your shoulders for playing the fish after the hookup.
Sincerely,
Salmo g.