Please add this as a feature request. Your competitors have this feature. It really would streamline the purchase process.
In the interim, is there an easy way I can intercept the launch of the product details page, perhaps to make that modal and reduce the number of paging required to order ? e.g., if I could wrapper the product detail page and make that modal, I could give the perception of a streamlined process (even though it really isn't). Anything to cut down the number of pages that are required to get a single product (with variants) to checkout. 5 pages is a lot of paging (catalog, prod page, go to checkout, sign-in/register, then finally checkout with payment). Collapsing to a 2-step process would really help. This topic would handle combining steps 1 & 2.