This case even applies to some GPUs at their default frequency though. GM204 (Hawaii is probably a better example) has plenty of memory bandwith to spare, while GP204 probably is slightly starved at default frequencies. That is something you would hide if you clocked the chips to an arbitrary lower frequency, but it is inherent to the product if you actually use it (GP204 at <1.5 GHz, GP204 at >>1.6 GHz).The IPC curve is not going to be inversely proportional to frequency at low frequencies where the effect of memories is irrelevant. That's pretty straightforward. It's also a completely uninteresting case.
Both Memory bandwith/clock speed and chip clock speed are equal parts of the design choice made by Nvidia and together they make up actual performance. Any discussion about IPC should reflect this since we can't buy the GPU with a different memory arrangement.
The Willamette vs. Tualatin vs. Thunderbird flashback is strong in this thread.