For those still following this thread, we’ve got a couple of extensions in this space.
Percentage Off Tiered Pricing allows you to set tier prices as a % discount - you can have both fixed price and % discount on a single product.
We also have Price Updater, which allows you to create templates of price information and apply to products. It works with Percentage Off Tiered Pricing so you can update many products at once.
Both extensions are available through the WebShopApps App Shop and there is more information on our website.