Update (19.05.2016): The bug was just fixed by Antti Koivisto and has landed in the current update of iOS (9.3.2) and Safari for OS X 9.1.1 (11601.6.17). When a user visits a site using a SVG sprite in a browser with an empty cache, the sprite is cached and will not be loaded multiple times any longer. You'll find more details here (in German), and Sven Wolfermann's results before and after the iOS update.
Using external SVG sprite maps to deliver lossless scalable vector images is widely used in responsive web design today and well-supported by tools like svg4everybody.
At German newspaper Zeit Online, we embraced this technique quite a lot. However, we recently changed this workflow back to completely inlining the SVG into the HTML owing to a bug in Apple's Safari browsers (mobile since iOS 9.3, in Mac OS X since Safari 9.1) – the same way GitHub is doing with its octicons.