The IHS isn't really relevant from a coverage POV, it's only there to shield the CPU from excessive pressure so idiots don't break it by overtightening the cooler (same as the four rubber bump stops on the Athlon XP). The CPU dies on TR cover a ~30x55mm area, and while the base plate on the NH-D15 is only ~46mm2 resulting in a 4.5mm underlap at the long edges, that won't really result in a measurable difference to if they were completely covered, because the IHS does cover them.
It's complicated but basically you are not going to end up with a situation where a amall % of the die is noticeably hotter than the rest when under a heat spreader that's under the contact plate of a gigantic heatpipe cooler. Yes the NH-D15 would perform better if it covered the complete die area, but we're talking less than a whole degree better. It's by no means cause to warrant not supporting TR4.