I don't know of a model C70. Do you mean C270? If so, I have T270 trimmers with the same engine. there's plenty of power and it always starts in 1 or 2 pulls. I've used saw blades, but not the tornado blade.
We use a stihl fs450 here at work. It is a complete monster. We only use the solid brush blade (triangle like) and it works great on clearing brush up to a inch or so. Anything bigger than a inch, it works, but is a bit slower. Often find a chainsaw is more effective for areas with 2" woody stuff.
As for string trimming with it, I'm not sure of the size of the shin., but its not very pleasant with the 450. Its just too big and heavy. You have to use the shoulder/belt harness, as it cannot be operated any other way. I have a string head for it, but it got used once, and now I'd be hard press to even find it.