Undertaker,
The GrandSpey works very well from 50 feet out to as far as you would probably ever want to cast when fishing. It turns over well at 50 feet and equally well at 100+ feet. It is certainly capable of casting and turning over large flies on type 6 and type 8 sink tips well beyond 100 ft. I fish my 10/11 GrandSpey out to 120+ feet with sink tips on certain runs on the Skagit. Although most of the time, I am fishing between 60 and 90 feet.
Another thing to keep in mind with the GrandSpey is because it is such a long line that carries a lot of grains of weight, you cannot overline your rod by a line size or you will seriously overload your rod and possibly break it from the strain. If you have a 9 weight (or 9/10 weight -except G.Loomis GLX) you need to use the 8/9 GrandSpey. Likewise if you have a 10 weight (or 10/11 weight-except G.Loomis) you need to use the 9/10 GrandSpey. The big boy GrandSpey 10/11 is reserved for the big rod cannons like the T&T 1511 and 1611, the Sage 10161, B&W 10-12 15 and 18 foot rods, G.Loomis 15 ft 10/11 and 15 1/2 ft 11/12 rods, etc. And Simon now has a 7/8 GrandSpey available for the 8 weight rods.
As much as I lovve the GrandSpey, I would not recommend the GrandSpey (or any other long belly spey line) to a beginner to spey casting because the temptation to try and lob one out there to 100 feet is too great; eventhough, the long belly lines are superb casters at 60 feet.
I recommend beginners buy the MidSpey because it is 65 feet of belly and that is as far as a beginner should be casting until s/he learns to spey cast well. And after learning how to cast the 65 feet of belly consistently, you can then shoot line until you get to 100+ feet.