I chopped all my Sedum Spectabilis last year, except one as a control. I have a lot of these plants around my large garden as the bees and butterflies love them. I was a bit apprehensive as to what would happen but they flowered beautifully and stayed upright and squat - unlike the unchopped plant which looked dreadful. I will see which plants benefit from the Chelsea Chop this year and do it again.
Other plants which respond well are:
Anthemis tinctoria (Golden marguerite)
Echinacea purpurea (Purple Coneflower)
Helenium (Sneezeweed)
Phlox paniculata (Garden Phlox)
Solidago (Goldenrod)