Sum and Substance can get pretty big, so I tried to come up with a variety of plants that can fill in to about that size, tolerate that kind of harsh afternoon sun, and still provide that golden/chartreuse foliage flair. I hope this helps!

Caryopteris x clandonensis 'Jason' (Sunshine Blue)
Cotinus coggygria 'Golden Spirit' (Ancot)
Forsythia x intermedia 'Fiesta'
Rhus typhina 'Bailtiger' (Tiger Eyes)
Spiraea japonica 'Gold Mound'
Persicaria amplexicaulis 'Golden Arrow'
Symphytum x uplandicum 'Axminster Gold' Russian Comfrey
Abelia x grandiflora 'Kaleidoscope'
Fallopia baldschuanica 'Lemon Lace'

Another possibility is to pair a Dicentra spectabilis 'Gold Heart' with a Caryopteris divaricata 'Snow Fairy'. While the Bleeding Heart is beginning to decline, the Caryopteris (which is usually late to start) is beginning to emerge so I like planting them very close together to keep that area from being bare. I think they're great companions, but I'm not sure if you'd consider the white/cream variegation of the Caryopteris to be golden enough in hue.

Best of luck! What a fun dilemna. :)

ps - sorry if any of my ideas don't fit your zone

Look up that recent thread where everyone posted their favorite gold plants. It had tons of ideas in there.

Good idea, echinaceamaniac. Alina, I liked both those threads and the sharp contrasts. Here you go, nydepot:

Now, what are your favorite golden foliage plants?
Your favorite black/dark red/maroon plant?