Miscanthus Purpurascens is actually not a sinensis.

If you've ever divided a clump of purpurascens, you know that it spreads with short growing rhizomes and divides much differently than sinensis...almost more like you'd find in a panicum grass. Purpurascens also has foliage that feels much different from most miscanthus varieties.

Don't worry about running, that 8" rhizome is about as long as you'll see...it'll still maintain a clump shape.

The problem is that at 8" each year it will get big fast. It does not seem to be making a very tight clump.

It's not going to spread 8" a year and it is a clumping grass. Unless you have the wrong plant, it should make a mature clump about 2 1/2 to 3' wide before it begins to see center die-out and dividing may become necessary.