Just updated our app Advanced Collection to include this feature - https://apps.shopify.com/advanced-custom-collections
You can create a collection that includes / excludes products which has a discount percentage (i.e. price difference between ‘price’ and ‘compared at price’, divided by ‘compared at price’) within a certain range that you choose. Whenever you add or update a product, the app will do a check and syncs that collection automatically.
Hope it helps and do try it out with a 7-day free trial =)