Matrixify

I would give this 10 if I could! We went live with 1500 skus and no way would it have been possible with the Shopify csv upload.
Support is absolutely amazing, Maris is responsive, thinks about what you are trying to achieve and is a delight to work with.
Tutorials are clear and extensive. Works with basic data plus metafields, all collections, Google data feeds...everything you can think of, even multiple image loading.
A must have app!