The CPU side is still on the same 22nm process and about as efficient and also needs to somehow fit into the 35/28W power envelope. The HD 4000 makes up some performance by having to play with 35W rather than the 28W on the Iris, so it runs almost always close to its peak performance while Iris on reaches its peak performance for short bursts and mostly is far far away from peak performance. Most of the Iris performance is purely theoretical.
The UEFI pre-boot firmware architecture (32-/64-bit) must match the GOP driver. Check if your rig uses GOP or legacy VBIOS. It pure performance the difference isn't all that big. Open Intel control panel / options / info-center. The Late 2013 has everything on one very power efficient chip, which means very low power consumption when it isn't doing much and less heat (it has only one fan because it doesn't need two anymore). The only reason the newer notebook is worth it is battery life. that your interest was in video editing and not in all kind of crap.
If they were on even drivers the difference would likely be smaller still. The new Macbook Pros come with both, an Intel HD4000 integrated processor and a. Most of the benchmarks on notebookcheck with the HD 4000 are also on older drivers.
Iris Pro is much faster (70%) because it does not have the bandwidth problem although it is otherwise the same GPU which cannot clock any higher. Generally the HD 5100 is a big GPU with lots of theoretical potential from which not much is left in real situations as the TDP the memory bandwitdh and the CPU side limit it too much. They range from about even to 30% difference sometimes twice as fast. You can compare real difference here by checking the HD 4000 and the HD 5100 respectively. People actually exagerate the difference between those GPUs.
Mac minis are not supported nor are any Mac running Intel HD integrated GPUs (HD3000/HD4000). You need a GPU that can do CUDA processing like a GeForce GTX card. Resolve won't run on that machine as the GPU is not supported by Resolve. The GPU won't make too much of a difference for you. Re: Intel HD 4000 768 MB & Davinci Resolve.