That's basically what I'd expect judging by PC benchmarks. Mobile cards are hard to compare because they're sitting in different laptops with different processors and different cooling systems, so I usually look at their desktop counterparts:
680MX/780M are both full-size GK104 chips found on GTX 680 and GTX 770. Desktop variants are clocked higher obviously, but practically that's all the difference.
M395 is a Tonga chip found in R9 285 and 380. Telling by available data, they also only differ in clock rate.
M395X is the full size version of M395. Doesn't have a desktop equivalent so far, would equal the (yet to be released) R9 380X.
Sitting in the same PC system, the Tonga cards (R9 285/380) are barely (if at all) faster than the 2012 era Nvidia Kepler cards. A 2015 Maxwell card in the same power envelope would virtually destroy them. AMD has still a quite good price-performance ratio with their Hawaii cards, but those 300W TDP beasts are obviously a no-go for the slim iMac.
Obviously this isn't 1:1 comparable to the mobile variants sitting in the iMacs, but it makes clear what you can expect from the 2015 AMD cards.
Of course it could be much worse (hello, 4K iMac!) and the performance isn't bad compared with gaming notebooks, but when I read Apple stating something like this:
I honestly don't know whether I should laugh or cry

I also don't understand how making the iMac as thin as possible increases the "ultimate desktop experience". There was nothing wrong with the old form factor, which I had myself some years ago.