I wouldn't use the workbooks alone for enrichment; if you had to do just one thing, you'd be better off teaching from just the textbook, or doing problem-solving enrichment with the Intensive Practice / Challenging Word Problems supplements. That said, Singapore Math has level-setting tests available to help assess a proper starting level. Good luck.

ETA: Have you checked out the Mathematics Enhancement Programme? It's free.

Striving to increase my rate of flow, and fight forum gloopiness. sick