We have a steeply terraced garden, and Lizzie27's suggestion of Stipa Gigantea is spot on. Ours looks amazing when the light catches them. Hakonechloa and Astelia would cope with both of those sites. Calamagrostis Karl Forster and Calamagrostis Brachytrycha, great grasses, but not as transparent as Stipa Gigantea. How about something to pick up the colour of your red Acer, like Cotinus, Eucomis Sparkling Burgundy, or perhaps some red/purple Heucheras. Steep terraces are a nightmare, but also thrilling because you can create real drama. I'm both sympathetic and envious!