Need a picture.

I doubt it will hurt this rumbustuous plant but it will change the appearance. How about doing the back half now and the front half next year? Then you'd not notice the cuts.

Sorbaria is a pretty tough shrub. My husband usually has to remove the many spreading roots each spring. It;s a great shrub for erosion control and it makes beautiful long stems for flower arrangements.

Yeah, it's a toughie. Go ahead.