3DMark Ice Storm Unlimited is CPU limited and not particularly stressful on the latest and greatest ultra mobile GPU's (the framerate is > 200fps in game test 1 and > 100fps in game test 2). GFXBench 3.0 on the other hand is GPU limited.
Okay, though now we also have 3DMark Cloud Gate results for the Core M tablet - surely that doesn't also fall under the category of not being adequately stressful? Sadly we can't make direct comparisons, but we easily can compared Core M to, say, a GTX 750M and then compare that to K1 Gfxbench. Namely, Core M obtains a Cloud Gate graphics score of 5737 versus an average, according to notebookcheck, of 11088 for a GTX 750M. So it's safe to say that Core M is roughly half the performance of a GTX 750M in that benchmark. Then we can easily compare a GTX 750M against the Shield Tablet under GfxBench - 92.2 fps vs 30.9 fps for Manhatten and 159.5 fps vs 64.7 fps for T-Rex. So 3x faster for Manhatten and 2.5x faster for T-Rex. Also, just to complete the comparisons to see how Intel's Gen7.5 at least translates performance between 3DMark Cloud Gate and Gfxbench, Iris Pro 5200 is basically equal to a GTX 750M on Cloud Gate but falls notably behind on Manhatten, 76.5 fps vs 92.2 fps, and slightly behind on T-Rex, 152.1 vs 159.5. If that trend continued then the end result would be that Core M would be about 1.2x faster than K1 in Gfxbench.
Which is the long way of saying that the information thus far points to Core M being quite competent in terms of graphics. Not as far ahead as Intel would probably like of course, but Intel has long since proven that they can't forecast graphics performance targets very well.