I've got the iMac 5k with the system maxed out to highest specs. Faster performance in the viewport then the standard iMac as I've used both though I haven't tested with huge scenes. Of course from cinebench scores the new iMac is more powerful but that screen is said to consume some graphics power just to run. Saying all this I recently installed boot camp on my machine and cinema 4d, because I needed to use a windows plugin - shocked I must say by the increase in performance in windows compared to Mac OSX on the same machine. On the Mac I have a scene with thousand of clones and high res textures and can barely navigate the scene with textures turned on whereas it is smooth as silk on windows.
Relax sir. I work in an FX studio and no-one here would ever consider PCs. We all work on maxed out iMacs. Anyway, I will try to keep an open mind if you do. Enjoy your PC. It's just a compooter after all.
How much speed to you need? Also for a beginner a MAC really is enough for all your needs. Plus as you buy more computers you may find net render will take care of all your rendering issues. And of course the screen on an imac or mac pro laptop will kick the out of a nasty low contrast PC screen and don't forget many other professionals are using the same screens to view your work. Also add into this OSX over windows, which is designed for graphics professionals and I really do not see the advantage of a slight speed bump.
id say you need some sort of SSS on the coffee material so you can see lighter/orangey shades where the light penetrates perhaps. Have you tried adding transparency and then adding dispersion to fake sss?