I have turned to this list but do not know which are the relevant retina iMac processors. Can you help please?
The new iMac refresh is using the i5-4590 as the base, the i5-4690 for the mid, and...er, some i7, I can't find out which. They're all Haswell based, and will all roughly have the same single core performance.
Really, the only thing you need to concern yourself with is if it's an i5 or an i7. Or rather, 4 cores vs 8 cores. For any multithreaded program that only takes advantage of 2 to 4 cores, you won't see much difference between an i5 or an i7. The i7 does have some bonuses that come with its higher clock speed and slightly bigger cache, but it won't make an earth shattering difference.
Now if you're using a program that can take advantage of all the cores it can get, or you're doing heavy multitasking, which is not just having a bunch of programs open, but are actively using them (IE rendering 3D in one program, while spicing together audio clip on another at the exact same time), this is where the i7 starts outpacing the i5. You have double the cores (or rather, the same amount of cores that can handle two threads at once, rather than just one), which means you can juggle more more gracefully, do one really big task that much quicker, or both.
Movie editing or 3D rendering will benefit more from an i7. An i5 isn't exactly sucky here, but it won't finish tasks quite as fast. For image editing and web design, an i5 will work just as well as an i7, since you'll rarely ever need more than 4 cores.