Will my Waratah (Shady Lady) grow more efficiently if i plant it in the ground or in a large container?

Yes, Waratah will always grow better when providing sufficient room to grow. You may try water in some 'Yates Dynamic Lifter Liquid Concentrate' as a gentle fertiliser and soil improver. It's certified for use in organic gardens, and contains seaweed, blood&bone and fish meal to help developing new roots and restoring healthy growth. Click here for more info on this product: https://www.yates.com.au/product/garden-fertilisers/certified-organics/yates-dynamic-lifter-liquid

Hope this helps and happy gardening.