But the real upgrade would be to models that either already had integrated graphics (and now will have much faster integrated graphics), or that had GPUs (and with the new pascal/polaris generations, will likely have much faster and less power hungry GPUs).
You might also notice improvements in SSDs, though I'm skeptical that despite large relative improvements most workloads really benefit much - after all, any semi decent SSD will have largely moved most bottlenecks out of the I/O subsystem, and then it really won't matter much if you reduce those last few milliseconds of latency even further. Some people will notice, however, and what you will notice is that you don't need to choose between fast or large as much anymore - a 1TB ssd is pretty affordable, and smaller SSDs might even be classified as "cheap" by macbook standards.
Still, I last updated (my non-mac laptop) just after haswell was released, and discounting SSD size and GPU, modern laptops seem very unattractive. Due to the glass screen fetish nowadays; most are heavier and more fragile to boot - my 13-inch, more than 3-year old laptop weighs 929g, and the battery still lasts many hours. It's fallen a few times and has a few nasty scratches and one nicked corner, but... upgrades still seem kind of pointless. Are you really going to upgrade for a crappy-but-less crappy iGFX or a larger SSD?